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The importance of birds in the diet of otter Lutra lutra on Shapwick Heath

The importance of birds in the diet of otter Lutra lutra on Shapwick Heath Volume 1 † Number 2 † June 2008 10.1093/biohorizons/hzn018 ......................................................................................................................................................................................................................................... Research article The importance of birds in the diet of otter Lutra lutra on Shapwick Heath Daniel Charles de la Hey* School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK. * Corresponding author: Tel: þ44 (0)7736674370. Email: danny_delahey@yahoo.co.uk Supervisor: Mr G. M. Smith, School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK. ........................................................................................................................................................................................................................................ This study investigates the importance of avian species in the diet of Shapwick Heath’s otter population. The frequency occurrence of prey items was recorded from spraints collected between July 2006 and January 2007. Bird feathers extracted from otter spraints were investigated under a microscope. Where possible, feathers were identified to species level. The presence of fish scales in spraints was recorded, but not quantified. Bird feathers were recorded in 41% of all spraints, compared with 4.6% on Slapton Ley in 1981 and 4.7% on the Somerset Levels in 1975. The highest proportion of bird feathers was recorded in July 2006, at 61%. Ralliformes were more frequently recorded than any other family of birds. Coot Fulica atra L. were more frequently recorded then any other species; 15 spraints were found to contain coot feathers. Mallard Anas platyrhynchos feathers were recorded in six spraints. Cormorant Phalacrocorax carbo feathers were found in three spraints and Water rail Rallus aquaticus feathers were found in two spraints. Pintail Anas acuta, teal Anas creca, moorhen Gallinula chloropus and little grebe Tachybaptus ruficollis feathers were found in one spraint each. Two spraints contained feathers from both mallard and coot, respectively. The high proportion of birds in the otter’s diet, in comparison with past studies suggests that otters are substituting birds for part of their traditional diet of fish. It is recommended that studies into the nature of the fish stock and the development of the aquatic ecosystem on Shapwick Heath be undertaken. Key words: Lutra lutra, Shapwick Heath, diet, ralliformes, spraint, feather. ........................................................................................................................................................................................................................................ Introduction predation on birds, in the case of Slapton Ley in the spring, may be due to vulnerable fledglings and adult birds sitting on Feeding strategy eggs, which are easier to catch than the fish. 1, 2 Previous studies have suggested that otters predate on small, The feeding habits of the Eurasian otter Lutra lutra immobile fish species with a high calorific value and a low 1–8 L. have been extensively researched. These studies have handling time in preference to large or highly agile prey. often overlooked or ignored the importance of individual While cubs are learning to become proficient hunters, it has bird species as a component in the otter’s diet. Chown been suggested by Kruuk that cubs favour prey that is suggested that the population of certain bird species, i.e. readily available, with the emphasis on ease of capture, coot Fulica atra L. and dabchick Tachybaptus ruficollis rather than the calorific value or shortest handling time. Pallas, on Shapwick Heath National Nature Reserve Perhaps the most important factor affecting rates of predation (NNR) have not reached the numbers that ecologists would is availability of food. Otters are commonly thought of as a normally have expected in an area that is of national import- species that predates solely on fish, but their diet includes a ance for wildfowl and wading birds. Chown implies that the wide range of species. The otter is therefore a generalist carni- poor reproductive success of bird species, in particular some vore that is able to capture prey as chance allows, rather than Ralliformes, may be the result of learned behaviour, i.e. pre- being a specialist piscivore as was commonly perceived. dation on birds, by the otter population. Chanin describes the differences in otter diet on Slapton Ley The purpose of this study is to investigate the claim that in Devon, 86 miles from the study site, being attributed to the otters on Shapwick Heath predate on the resident wild- seasonal changes in the activity of the prey. The increased fowl population to a higher degree than previously reported. ......................................................................................................................................................................................................................................... 2008 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 143 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... It is intended to establish if certain species of birds are Spraint contents identification favoured for predation by otters and if any seasonal variation After air-drying the spraints, they were placed in a solution in the predation rates can be established. of water and detergent (5:1) for a minimum of 2 h. Tweezers were used to help break up the spraints and feath- ers were removed from the detergent solution and washed in Methods fresh detergent and water solution. The feathers were rinsed in water and then alcohol, before being left overnight to dry For the purpose of this study, diet was assessed using the out; dry feathers being easier to prepare than wet ones. contents of otter faeces. The use of spraint analysis to deter- Preparation of the feathers under the microscope was mine otter diet is considered a viable method and has been adopted from Day. A single barb was cut from the rachis 2, 7, 8, 10 – 12 used frequently before. The methods for spraint using a scalpel, mounted on a slide and held with a drop identification have normally been used to identify the of alcohol. To prevent the barbules from closing together species of fish in otters’ diet; the identification of bird feath- and remain fanned out; a cover slip was aligned with the ers has previously only been undertaken to family level. barbule from the proximal end to the distal end. Identification of bird remains has in the past been carried Identification from a single barbule was not always possible. out on the stomach content of stoats and this has been If this occurred, a second barb was removed and the process adapted and expanded on to enable identification of birds repeated, until identification could be made. to species level from otter spraints. The barbules were examined using a binocular Nikon Labphoto 2 light microscope, and findings were cross- The study site referenced with information from Day and from the Bird Remains Identification System (BRIS). Shapwick Heath was created from flooded peat workings that have become a habitat well suited to wetland birds. Shapwick Heath is designated a Site of Special Scientific Interest (SSSI) and a Special Protection Area (SPA). It is Results part of the Somerset Levels Wetlands Ramsar Site and a A total of 87 spraints were analysed from Shapwick Heath from National Nature Reserve. The site is owned and managed July 2006 through January 2007. They were collected and ana- by Natural England. Shapwick Heath has 394 acres in lysed using procedures outlined in the Methods. As the number area, consisting of a mixture of eutrophic open water of spraints found present per month varied, so the number ana- habitat, reedbed, fenland and meadows. lysed varied. The majority were collected from the bridge at British Ordnance survey grid reference ST 449, 397. Spraint collection Due to the structure of the bridge, the spraints remained Otter spraints were collected on Shapwick Heath National out of the water all year round and the bridge structure pre- Nature Reserve between July 2006 and January 2007. vented them from being washed away by rain. The bridge at Spraints were collected from various sites along the main OS ST 423, 411 at Ashcott Corner was less useful as the path that runs from east to west of Shapwick Heath water level inundated the ledges under the bridge, making Nature Reserve OS ST 449, 397 and OS ST 423, 411. The it impassable for otters at times of peak flow. During low path allows year round access to the sprainting sites. flows, waterfowl often rested on the ledge under the bridge Spraints were identified by their distinctive appearance, and contaminated the otter spraints by defecating on them. often containing visible feather and fish scales. For these reasons, the bridge at Ashcott Corner was often In addition to visual signs, spraints were identifiable by unsuitable for collecting spraints. A spatial analysis of the their characteristic smell. Spraints were only collected site was attempted. However, due to the nature of the collec- from areas showing evidence of use by otters, such as runs tion site, an unbiased result for a spatial analysis of the site in the vegetation on the drain banks, which displayed otter was not possible. The majority of the spraints were collected footprints. Before collection began, all spraints were under bridges. This is likely to represent the amount of time removed from the site so that a clear time-scale could be that a spraint survives, rather than demonstrate a difference given as to when the faeces were produced. in the levels of predation in different areas of the reserve. Gloves were worn at all times when handling spraints to Figure 1 shows the monthly trend in prey items found in prevent infection. Spraints were stored individually in otter spraints. The remains of fish have been found more fre- freezer bags that were labelled with: a record of the location quently than any other prey remains. In July 2006, the per- of where the spraint was found, the date and comments on centage of fish remains as same as bird feathers found in the condition of the spraint, i.e. its freshness. While collect- the otter spraints, being 60% and 61%, respectively. In ing the spraints, the presence of wildfowl species on the August, the amount of spraints containing fish remains was reserve and those flying over it were recorded. consistent with July, amounting to 61%, but bird remains ......................................................................................................................................................................................................................................... 144 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... such as dead trees at the Roughet. Juvenile birds were often seen closer to the water than adult birds. Mute swans and herons were often observed within a few metres of an otter in the water at West Meare Heath. Discussion The importance of fish in otter diets has been widely published, recorded with fish the highest-ranking component in the otters’ 1, 5, 6 diet. On Shapwick there appears a markedly smaller 1, 5 reliance on fish as a food as compared with similar studies. The majority of studies on otter diet have investigated the type of fish that have been taken by otters. This study looked at the frequency occurrence of fish remains found in otter spraint. It did not attempt to quantify which species were predated upon. The results showed that 67% fish remains Figure 1. The percentage change of spraints content through time using in otter diet on Shapwick Heath shows a similarity with data from 87 spraints, which were collected from July 2006 to January 2007 the European otter populations, in that they are less reliant on Shapwick Heath. The number of spraints collected each month is shown on fish than on the diet shown in studies of the British in brackets. Percentage distribution is used to allow for the variation in the 3, 6 otter population. number of spraints collected throughout the study period. If the assumption that otters feed on the prey items as the chance arises is correct then there may be a number of were found in only 19% of the spraints. The percentage of reasons for a smaller reliance on fish by the Shapwick Heath spraints with fish remains fell during August and otter population may be a number of reasons; there may be a September from 60% to 50% and the number of spraints less well-formed aquatic ecosystem as a result of low pH, containing bird feathers increased to 38%. The spraints col- which is due to the high peat content of the soil. Acid water lected in October were recorded as having the third largest results in fewer invertebrates and generally has sparser and percentage of fish remains in them, being 65% and the per- less complex aquatic vegetation than water with a higher centage of spraints that had feathers in them rose to 50%. pH. There has been a reported decline in the freshwater November saw the percentage of spraints with fish rising fish population forcing the otter to eat other prey items. again to 85%. The amount of bird feather remains in The decline of an important prey item such as eel may November declined, with only 32% of the spraints collected have had a knock-on effect of forcing the otter to change and analysed containing bird feathers. December saw the its foraging behaviour, so that time is not wasted searching lowest recorded percentage of fish during the monitoring for a prey item that is no longer present in large numbers. period, only 20% of the analysed spraints had evidence of Therefore, it is feasible that otter population on Shapwick fish scales, the remains of birds feathers was recorded as Heath has adapted its behaviour, so that hunting birds 50%. From the spraints collected in January 2007, there have become a larger part of foraging practice. was evidence of fish scales in 67% of the sampled spraints, The findings on Shapwick Heath follows a pattern that is and 50% of the spraints had bird remains. similar to those described by other European studies, Table 1 shows observed behavioural responses of wildfowl though in Europe it is often crayfish rather than birds that species in relation to otters. Black-headed gulls were are taken in larger numbers. The percentage of fish in observed displaying mobbing behaviour in the presence of Shapwick Heath otters’ diet is lower than that may have otters. They were sighted hovering over a bed of been expected, when compared with other studies, such as Phragmites, following the progress of an otter that McMahon and McCafferty. These authors found that fish emerged from the reedbed. The mobbing behaviour contin- scales were present in up to 90% of the otter spraints. On ued after the otter had entered the water. Moorhens were Shapwick Heath 67% of the spraints showed evidence of fish observed in the backwater of the nature reserve in smaller scales. There were temporal variations in the percentage of areas of water that was well vegetated. Moorhens were fish scales found in the otter spraints. In October 2006, also observed foraging on dry land on the reserve. Coots scales were found in 80% of the spraints but this fell in were observed foraging in large groups, often in 20 or December to 20%. The results imply that the otter population more, in areas of open water in the Roughet and West relies on fish for about 60% of their diet. The rest of the energy Meare Heath. They were observed resting on the water needs are made up by substituting birds for fish, and to some edge in West Meare Heath. Cormorants were observed at extent other prey items. December had a very low percentage the end of the day resting on protrusions from the water of fish remains found in the sample. This month also had the ......................................................................................................................................................................................................................................... 145 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... Table 1. Bird behavioural activity observed during the survey Species Date Activity Relation to otter activity ........................................................................................................................................................................................................................................ Black-headed gull 12 August Mobbing an otter when in reeds Phragmites australis and Number of birds aggressive to otter larus 2006 when in water at West Meare Heath Coot All year Intraspecific competition whilst on open water. Sheltering in Foraging in open water. Intra- and interspecific exposed areas of the Meare Heath aggression. Ground nesting bird Moorhen All year Foraging amongst vegetation on the land Foraging behaviour on land and nesting on the ground Cormorant All year Distance from water when drying wings Often the juvenile birds were observed perching near to water Mute swan All year Foraging Unconcerned by presence of otters Grey heron All year Foraging Unconcerned by presence of otters All behavioural aspects are intended to show the relationship between otter and wildfowl on Shapwick Heath. lowest total spraint collections, just six spraints, which may species, which increases the risk of predation. Chanin have skewed the accuracy of the results for December. The pro- suggested the reason for higher indices of predation, when portion of birds predated upon by otters is much larger than in comparing ducks to Ralliformes, is because ducks roost on 3 6 other similar studies. Chanin and Mason and Macdonald open water. This is suggesting that otters do the majority recorded the highest previous percentage of 10.3% from of their foraging nocturnally, catching ducks in the open Blakeney, but the Shapwick Heath otter population in 2006 while they are unaware. Ducks were as common as coot on appears to be predating up to four times that amount. Webb all areas of Shapwick Heath, and Ralliformes were as numer- investigated otters’ diet on the Somerset Levels and recorded ous as ducks on Slapton Ley. Therefore, a difference in similar levels to Chanin, which were 4.7% and 4.6%, respect- population density cannot explain the different ratio in pre- 4 3 ively. When compared with Webb and Chanin the results dation on the two sites. of this study suggest that Shapwick Heath otters could be Ferraras and Macdonald investigated the rates of preda- eating 10 times the amount of birds that have been previously tion on waterfowl by mink. The study found that mink reported. predate on coot to a far greater frequency than on moorhen. This study supports the theory that otters are predating The reason for this has been suggested to be the difference in certain wildfowl species at a higher proportion than has roosting behaviour between the two bird species. Coots been expected. The peak period of predation on birds was roost on the extremity of the water margins, whereas July when 60% of the spraints had bird feathers in them, moorhen roost in deeper, thicker part of vegetation. and coincides with the breeding season of many wildfowl Moorhen often build their nest on the top of logs, which lifts species, when fledlings and adult birds are considered to be them out of the water. If this adaptation helps reduce preda- 18 18, 20 more vulnerable. August was the month with the lowest tion from mink which is much more agile and capable of recorded percentage of bird feathers, being 20%. The climbing, it would seem likely that by lifting their nest out of reason for this low level is unknown. thewater, moorhens also reduce the riskof predation byotters. Chown speculated that the coot population might be Cormorant feathers were found in three spraintstwice in July lower than expected in the reserve as a result of predation and once in August. The presence of cormorant feathers in the pressure from otters. There were 16 cases of analysed spraints for that part of the year suggests that otters were pre- spraint samples containing coot feathers. This was the dating on inexperienced juveniles. Young birds are much most frequently recorded wildfowl species in the study. more likely to be predated on as they lack awareness that is Coots are ground-nesting birds, which construct their nests gained through experience. Cormorants are a very common on the edge of the water, often in the reeds. During the sight on Shapwick Heath and they were observed every period of the study, coots were often seen resting on the month during the study. Cormorants have wettable plumage edge between the phragmites beds on open water ( personal that requires drying out after diving intowater and it is also the- observation). This behaviour makes coots and their fledgling orized that cormorants sunbathe to allow large meals to 20 21 vulnerable to otters and mink predation. digest. It was observed that the cormorants often stood on Coots feed on open water, and while feeding they are objects that protruded from the water. Juvenile cormorants very vocal and spend a lot of time displaying aggressive were seen perched on objects that were only just protruding behaviour. Aggressive interaction with conspecifics is an from the water ( personal observation). This social exclusion energy-reducing activity; it reduces time vigilance in quarry may increase predation risk in juvenile cormorants. ......................................................................................................................................................................................................................................... 146 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... 3. Chanin P (1981) The diet of otter and it relations with the feral mink in two Some birds associate otters as a threat to themselves and their areas of Southwest England. Acta Theriol 26: 83–95. nests. Black-headed gull were seen displaying mobbing beha- 4. Webb JB (1975) Food of the otter (Lutra lutra L.) on the Somerset Levels. viour towards an otter on 12 August 2006. While collecting J Zool 177: 486–489. spraints, three black-headed gulls were observed flying up and 5. Jenkins D, Harper RJ (1980) Ecology of otters in Northern Scotland: analysis down a bed of phragmites.Anotter emergedfrom the of otter (Lutra lutra) and mink (Mustela vision) faeces from Deeside N.E. reedbed and entered the water, the gulls continued to mob the Scotland 1977–1978. J Appl Ecol 49: 737–754. otter. Clode et al. investigated the mobbing behaviour of 6. Mason CF, MacDonald SM (1986) Otters Ecology and Conservation. black-headed gull and terns (Sterna spp.). Their findings Cambridge: Cambridge University Press. suggest that birds offer differing responses to predators. Adult 7. Lanszki J, Sallai Z (2006) Comparison of the feeding habits of Eurasian otters on a fast flowing river and its backwater habitats. Mamm Biol. 71: 346–366. birds hover over animals that are potential threat to them, 8. McMahon J, McCafferty DJ (2006) Distribution and diet of otters (Lutra lutra) which is classed as a fleeing response. Birds that feel their eggs in marine areas of Loch Lomond and The Trossachs National Park, Scotland, and young are threatened often offer an aggressive response to UK. Lutra 49: 29–36. the predator. They will attempt to drive the predator away 9. Chown D (2005) Shapwick Heath NNR: Waterfowl Populations and Breeding from the young, by repeatedly diving towards the predator. Success in 2004, and Comparison with 2002 and 2003. Peterborough: This aggressive response to potential predators carries the English Nature. risks of death or injury to the mobbing adult birds, so much 10. Jenkins D (1980) Ecology of otters in Northern Scotland: I. Otter (Lutra lutra) so that the black-headed gulls chose the fleeing type of breeding and dispersion in Mid-Deeside, Aberdeenshire in 1974–1979. J Anim Ecol 49: 713–35. mobbing. Mobbing behaviour is learned through experience 11. Jenkins D, Burrows GO (1980) Ecology of otters in Northern Scotland III. The possibly implying that otters have attempted to predate upon use of faeces as indicators of otter (Lutra lutra) density and distribution. J black-headed gulls and that otters are now perceived as a Appl Ecol 49: 755–774. threat by black-headed gulls on Shapwick Heath. Though 12. Bas N, Jenkins D, Rothery P (1984) Ecology of otters in Northern Scotland: II. The there was no evidence in the spraints of gulls being taken by distribution of otter (Lutra lutra) faeces in relation to bank side vegetation on otters, the adaptation of anti-predator by black-headed gulls the River Dee in summer 1981. J Appl Ecol 21: 507–513. suggests that they are predated on. 13. Day MG (1966) Identification of hair and feather remains in the gut and Otters are not perceived as threats by all birds. Grey faeces of stoats and weasels. J Zool 148: 201–207. herons and mute swans took no notice of otters in water 14. Conroy JWH, Watt J, Webb JB et al. (2005) A Guide to the Identification of the Prey Remains in Otter Spraint. London: The Mammal Society. near to them, which may be due to the bird’s larger size. 15. Prast W, Shamoun J (2001) Bird Remain Identification System (BRIS). Amsterdam: Grey herons have been reported as prey items of otter in 23, 24 Expert Centre for Taxonomic Identification, University of Amsterdam. Spain and suggested that the otters are killing the 16. Elmberg J, Sjo¨ berg K, Nummi P et al. (2004) Patterns of lake acidity and heron as an act of super-predation rather than killing the waterfowl communities. Hydrobiologia 279: 201–206. herons to consume them. 17. Knights B (2003) A review of the possible impacts of long-term oceanic and Therefore, it may be concluded from this study that the climate changes and fishing mortality on recruitment of anguillid eels of the otters on Shapwick Heath demonstrate a larger proportion Northern Hemisphere. Sci Total Environ 310: 237–244. of birds in otters’ diet than that from previous studies. 18. Ferraras P, Macdonald DW (1999) The impact of mink (Mustela vision) on Whether this is due to the particularly high local availability water birds in upper Thames. J Appl Ecol 36: 701–708. of birds as a food source at Shapwick Heath and whether this 19. Irwin J, O’Halloran O (1997) The wintering behavior of coot Fulica atra at Cork Lough, South West Ireland. Biology and the Environment. Proc R Irish level of predation impacts significantly on the overall bird Symp 97B: 157–162. population levels would require further research. 20. Macdonald DW, Harrington LA (2003) The American Mink; the triumph and tragedy of adaptation out context. NZ J Zool 30: 421–441. Acknowledgements 21. Gre´millet D, Wilson RP (1999) A life in the fast lane: energetics and foraging strategies of the great cormorant. Behav Ecol 5: 516–524. The author would like to thank James Williams of the 22. Clode D, Birks JDS, Macdonald DW (2000) The influence of risk and vulner- Somerset Otter Group for advice concerning otter spraints. ability on predator mobbing by terns (Sterna spp.) and gulls (Larus spp.). J Zool 252: 53–59. 23. Ruiz-Olmo J, Marsol R (2002) New information on the predation of fish References eating birds by the Eurasian otter (Lutra lutra) IUCN Otter spec. Group Bull 19: 103–106. 1. Carrs DN (1995) The foraging behaviour and feeding ecology of the otter (Lutra lutra): a selective review. Hystrix 7: 179–194. 24. Ruiz-Olmo J, Marsol R, Asensio V (2003) More on predation of fish eating birds by Eurasian otters (Lutra lutra) in captivity. IUCN Otter spec. Group 2. Kruuk H (2006) Otters, Ecology, Behaviour and Conservation. Oxford: Oxford Bull 20: 73. University Press. ........................................................................................................................................................................................................................................ Submitted on 1 October 2007; accepted on 28 January 2008; advance access publication 23 April 2008 ......................................................................................................................................................................................................................................... http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Bioscience Horizons Oxford University Press

The importance of birds in the diet of otter Lutra lutra on Shapwick Heath

de la Hey, Daniel Charles
Bioscience Horizons , Volume 1 (2) – Jun 23, 2008
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Volume 1 † Number 2 † June 2008 10.1093/biohorizons/hzn018 ......................................................................................................................................................................................................................................... Research article The importance of birds in the diet of otter Lutra lutra on Shapwick Heath Daniel Charles de la Hey* School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK. * Corresponding author: Tel: þ44 (0)7736674370. Email: danny_delahey@yahoo.co.uk Supervisor: Mr G. M. Smith, School of Science and the Environment, Bath Spa University, Newton Park Campus, Newton St. Loe, Bath, BA2 9BN, UK. ........................................................................................................................................................................................................................................ This study investigates the importance of avian species in the diet of Shapwick Heath’s otter population. The frequency occurrence of prey items was recorded from spraints collected between July 2006 and January 2007. Bird feathers extracted from otter spraints were investigated under a microscope. Where possible, feathers were identified to species level. The presence of fish scales in spraints was recorded, but not quantified. Bird feathers were recorded in 41% of all spraints, compared with 4.6% on Slapton Ley in 1981 and 4.7% on the Somerset Levels in 1975. The highest proportion of bird feathers was recorded in July 2006, at 61%. Ralliformes were more frequently recorded than any other family of birds. Coot Fulica atra L. were more frequently recorded then any other species; 15 spraints were found to contain coot feathers. Mallard Anas platyrhynchos feathers were recorded in six spraints. Cormorant Phalacrocorax carbo feathers were found in three spraints and Water rail Rallus aquaticus feathers were found in two spraints. Pintail Anas acuta, teal Anas creca, moorhen Gallinula chloropus and little grebe Tachybaptus ruficollis feathers were found in one spraint each. Two spraints contained feathers from both mallard and coot, respectively. The high proportion of birds in the otter’s diet, in comparison with past studies suggests that otters are substituting birds for part of their traditional diet of fish. It is recommended that studies into the nature of the fish stock and the development of the aquatic ecosystem on Shapwick Heath be undertaken. Key words: Lutra lutra, Shapwick Heath, diet, ralliformes, spraint, feather. ........................................................................................................................................................................................................................................ Introduction predation on birds, in the case of Slapton Ley in the spring, may be due to vulnerable fledglings and adult birds sitting on Feeding strategy eggs, which are easier to catch than the fish. 1, 2 Previous studies have suggested that otters predate on small, The feeding habits of the Eurasian otter Lutra lutra immobile fish species with a high calorific value and a low 1–8 L. have been extensively researched. These studies have handling time in preference to large or highly agile prey. often overlooked or ignored the importance of individual While cubs are learning to become proficient hunters, it has bird species as a component in the otter’s diet. Chown been suggested by Kruuk that cubs favour prey that is suggested that the population of certain bird species, i.e. readily available, with the emphasis on ease of capture, coot Fulica atra L. and dabchick Tachybaptus ruficollis rather than the calorific value or shortest handling time. Pallas, on Shapwick Heath National Nature Reserve Perhaps the most important factor affecting rates of predation (NNR) have not reached the numbers that ecologists would is availability of food. Otters are commonly thought of as a normally have expected in an area that is of national import- species that predates solely on fish, but their diet includes a ance for wildfowl and wading birds. Chown implies that the wide range of species. The otter is therefore a generalist carni- poor reproductive success of bird species, in particular some vore that is able to capture prey as chance allows, rather than Ralliformes, may be the result of learned behaviour, i.e. pre- being a specialist piscivore as was commonly perceived. dation on birds, by the otter population. Chanin describes the differences in otter diet on Slapton Ley The purpose of this study is to investigate the claim that in Devon, 86 miles from the study site, being attributed to the otters on Shapwick Heath predate on the resident wild- seasonal changes in the activity of the prey. The increased fowl population to a higher degree than previously reported. ......................................................................................................................................................................................................................................... 2008 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 143 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... It is intended to establish if certain species of birds are Spraint contents identification favoured for predation by otters and if any seasonal variation After air-drying the spraints, they were placed in a solution in the predation rates can be established. of water and detergent (5:1) for a minimum of 2 h. Tweezers were used to help break up the spraints and feath- ers were removed from the detergent solution and washed in Methods fresh detergent and water solution. The feathers were rinsed in water and then alcohol, before being left overnight to dry For the purpose of this study, diet was assessed using the out; dry feathers being easier to prepare than wet ones. contents of otter faeces. The use of spraint analysis to deter- Preparation of the feathers under the microscope was mine otter diet is considered a viable method and has been adopted from Day. A single barb was cut from the rachis 2, 7, 8, 10 – 12 used frequently before. The methods for spraint using a scalpel, mounted on a slide and held with a drop identification have normally been used to identify the of alcohol. To prevent the barbules from closing together species of fish in otters’ diet; the identification of bird feath- and remain fanned out; a cover slip was aligned with the ers has previously only been undertaken to family level. barbule from the proximal end to the distal end. Identification of bird remains has in the past been carried Identification from a single barbule was not always possible. out on the stomach content of stoats and this has been If this occurred, a second barb was removed and the process adapted and expanded on to enable identification of birds repeated, until identification could be made. to species level from otter spraints. The barbules were examined using a binocular Nikon Labphoto 2 light microscope, and findings were cross- The study site referenced with information from Day and from the Bird Remains Identification System (BRIS). Shapwick Heath was created from flooded peat workings that have become a habitat well suited to wetland birds. Shapwick Heath is designated a Site of Special Scientific Interest (SSSI) and a Special Protection Area (SPA). It is Results part of the Somerset Levels Wetlands Ramsar Site and a A total of 87 spraints were analysed from Shapwick Heath from National Nature Reserve. The site is owned and managed July 2006 through January 2007. They were collected and ana- by Natural England. Shapwick Heath has 394 acres in lysed using procedures outlined in the Methods. As the number area, consisting of a mixture of eutrophic open water of spraints found present per month varied, so the number ana- habitat, reedbed, fenland and meadows. lysed varied. The majority were collected from the bridge at British Ordnance survey grid reference ST 449, 397. Spraint collection Due to the structure of the bridge, the spraints remained Otter spraints were collected on Shapwick Heath National out of the water all year round and the bridge structure pre- Nature Reserve between July 2006 and January 2007. vented them from being washed away by rain. The bridge at Spraints were collected from various sites along the main OS ST 423, 411 at Ashcott Corner was less useful as the path that runs from east to west of Shapwick Heath water level inundated the ledges under the bridge, making Nature Reserve OS ST 449, 397 and OS ST 423, 411. The it impassable for otters at times of peak flow. During low path allows year round access to the sprainting sites. flows, waterfowl often rested on the ledge under the bridge Spraints were identified by their distinctive appearance, and contaminated the otter spraints by defecating on them. often containing visible feather and fish scales. For these reasons, the bridge at Ashcott Corner was often In addition to visual signs, spraints were identifiable by unsuitable for collecting spraints. A spatial analysis of the their characteristic smell. Spraints were only collected site was attempted. However, due to the nature of the collec- from areas showing evidence of use by otters, such as runs tion site, an unbiased result for a spatial analysis of the site in the vegetation on the drain banks, which displayed otter was not possible. The majority of the spraints were collected footprints. Before collection began, all spraints were under bridges. This is likely to represent the amount of time removed from the site so that a clear time-scale could be that a spraint survives, rather than demonstrate a difference given as to when the faeces were produced. in the levels of predation in different areas of the reserve. Gloves were worn at all times when handling spraints to Figure 1 shows the monthly trend in prey items found in prevent infection. Spraints were stored individually in otter spraints. The remains of fish have been found more fre- freezer bags that were labelled with: a record of the location quently than any other prey remains. In July 2006, the per- of where the spraint was found, the date and comments on centage of fish remains as same as bird feathers found in the condition of the spraint, i.e. its freshness. While collect- the otter spraints, being 60% and 61%, respectively. In ing the spraints, the presence of wildfowl species on the August, the amount of spraints containing fish remains was reserve and those flying over it were recorded. consistent with July, amounting to 61%, but bird remains ......................................................................................................................................................................................................................................... 144 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... such as dead trees at the Roughet. Juvenile birds were often seen closer to the water than adult birds. Mute swans and herons were often observed within a few metres of an otter in the water at West Meare Heath. Discussion The importance of fish in otter diets has been widely published, recorded with fish the highest-ranking component in the otters’ 1, 5, 6 diet. On Shapwick there appears a markedly smaller 1, 5 reliance on fish as a food as compared with similar studies. The majority of studies on otter diet have investigated the type of fish that have been taken by otters. This study looked at the frequency occurrence of fish remains found in otter spraint. It did not attempt to quantify which species were predated upon. The results showed that 67% fish remains Figure 1. The percentage change of spraints content through time using in otter diet on Shapwick Heath shows a similarity with data from 87 spraints, which were collected from July 2006 to January 2007 the European otter populations, in that they are less reliant on Shapwick Heath. The number of spraints collected each month is shown on fish than on the diet shown in studies of the British in brackets. Percentage distribution is used to allow for the variation in the 3, 6 otter population. number of spraints collected throughout the study period. If the assumption that otters feed on the prey items as the chance arises is correct then there may be a number of were found in only 19% of the spraints. The percentage of reasons for a smaller reliance on fish by the Shapwick Heath spraints with fish remains fell during August and otter population may be a number of reasons; there may be a September from 60% to 50% and the number of spraints less well-formed aquatic ecosystem as a result of low pH, containing bird feathers increased to 38%. The spraints col- which is due to the high peat content of the soil. Acid water lected in October were recorded as having the third largest results in fewer invertebrates and generally has sparser and percentage of fish remains in them, being 65% and the per- less complex aquatic vegetation than water with a higher centage of spraints that had feathers in them rose to 50%. pH. There has been a reported decline in the freshwater November saw the percentage of spraints with fish rising fish population forcing the otter to eat other prey items. again to 85%. The amount of bird feather remains in The decline of an important prey item such as eel may November declined, with only 32% of the spraints collected have had a knock-on effect of forcing the otter to change and analysed containing bird feathers. December saw the its foraging behaviour, so that time is not wasted searching lowest recorded percentage of fish during the monitoring for a prey item that is no longer present in large numbers. period, only 20% of the analysed spraints had evidence of Therefore, it is feasible that otter population on Shapwick fish scales, the remains of birds feathers was recorded as Heath has adapted its behaviour, so that hunting birds 50%. From the spraints collected in January 2007, there have become a larger part of foraging practice. was evidence of fish scales in 67% of the sampled spraints, The findings on Shapwick Heath follows a pattern that is and 50% of the spraints had bird remains. similar to those described by other European studies, Table 1 shows observed behavioural responses of wildfowl though in Europe it is often crayfish rather than birds that species in relation to otters. Black-headed gulls were are taken in larger numbers. The percentage of fish in observed displaying mobbing behaviour in the presence of Shapwick Heath otters’ diet is lower than that may have otters. They were sighted hovering over a bed of been expected, when compared with other studies, such as Phragmites, following the progress of an otter that McMahon and McCafferty. These authors found that fish emerged from the reedbed. The mobbing behaviour contin- scales were present in up to 90% of the otter spraints. On ued after the otter had entered the water. Moorhens were Shapwick Heath 67% of the spraints showed evidence of fish observed in the backwater of the nature reserve in smaller scales. There were temporal variations in the percentage of areas of water that was well vegetated. Moorhens were fish scales found in the otter spraints. In October 2006, also observed foraging on dry land on the reserve. Coots scales were found in 80% of the spraints but this fell in were observed foraging in large groups, often in 20 or December to 20%. The results imply that the otter population more, in areas of open water in the Roughet and West relies on fish for about 60% of their diet. The rest of the energy Meare Heath. They were observed resting on the water needs are made up by substituting birds for fish, and to some edge in West Meare Heath. Cormorants were observed at extent other prey items. December had a very low percentage the end of the day resting on protrusions from the water of fish remains found in the sample. This month also had the ......................................................................................................................................................................................................................................... 145 Research article Bioscience Horizons † Volume 1 † Number 2 † June 2008 ......................................................................................................................................................................................................................................... Table 1. Bird behavioural activity observed during the survey Species Date Activity Relation to otter activity ........................................................................................................................................................................................................................................ Black-headed gull 12 August Mobbing an otter when in reeds Phragmites australis and Number of birds aggressive to otter larus 2006 when in water at West Meare Heath Coot All year Intraspecific competition whilst on open water. Sheltering in Foraging in open water. Intra- and interspecific exposed areas of the Meare Heath aggression. Ground nesting bird Moorhen All year Foraging amongst vegetation on the land Foraging behaviour on land and nesting on the ground Cormorant All year Distance from water when drying wings Often the juvenile birds were observed perching near to water Mute swan All year Foraging Unconcerned by presence of otters Grey heron All year Foraging Unconcerned by presence of otters All behavioural aspects are intended to show the relationship between otter and wildfowl on Shapwick Heath. lowest total spraint collections, just six spraints, which may species, which increases the risk of predation. Chanin have skewed the accuracy of the results for December. The pro- suggested the reason for higher indices of predation, when portion of birds predated upon by otters is much larger than in comparing ducks to Ralliformes, is because ducks roost on 3 6 other similar studies. Chanin and Mason and Macdonald open water. This is suggesting that otters do the majority recorded the highest previous percentage of 10.3% from of their foraging nocturnally, catching ducks in the open Blakeney, but the Shapwick Heath otter population in 2006 while they are unaware. Ducks were as common as coot on appears to be predating up to four times that amount. Webb all areas of Shapwick Heath, and Ralliformes were as numer- investigated otters’ diet on the Somerset Levels and recorded ous as ducks on Slapton Ley. Therefore, a difference in similar levels to Chanin, which were 4.7% and 4.6%, respect- population density cannot explain the different ratio in pre- 4 3 ively. When compared with Webb and Chanin the results dation on the two sites. of this study suggest that Shapwick Heath otters could be Ferraras and Macdonald investigated the rates of preda- eating 10 times the amount of birds that have been previously tion on waterfowl by mink. The study found that mink reported. predate on coot to a far greater frequency than on moorhen. This study supports the theory that otters are predating The reason for this has been suggested to be the difference in certain wildfowl species at a higher proportion than has roosting behaviour between the two bird species. Coots been expected. The peak period of predation on birds was roost on the extremity of the water margins, whereas July when 60% of the spraints had bird feathers in them, moorhen roost in deeper, thicker part of vegetation. and coincides with the breeding season of many wildfowl Moorhen often build their nest on the top of logs, which lifts species, when fledlings and adult birds are considered to be them out of the water. If this adaptation helps reduce preda- 18 18, 20 more vulnerable. August was the month with the lowest tion from mink which is much more agile and capable of recorded percentage of bird feathers, being 20%. The climbing, it would seem likely that by lifting their nest out of reason for this low level is unknown. thewater, moorhens also reduce the riskof predation byotters. Chown speculated that the coot population might be Cormorant feathers were found in three spraintstwice in July lower than expected in the reserve as a result of predation and once in August. The presence of cormorant feathers in the pressure from otters. There were 16 cases of analysed spraints for that part of the year suggests that otters were pre- spraint samples containing coot feathers. This was the dating on inexperienced juveniles. Young birds are much most frequently recorded wildfowl species in the study. more likely to be predated on as they lack awareness that is Coots are ground-nesting birds, which construct their nests gained through experience. Cormorants are a very common on the edge of the water, often in the reeds. During the sight on Shapwick Heath and they were observed every period of the study, coots were often seen resting on the month during the study. Cormorants have wettable plumage edge between the phragmites beds on open water ( personal that requires drying out after diving intowater and it is also the- observation). This behaviour makes coots and their fledgling orized that cormorants sunbathe to allow large meals to 20 21 vulnerable to otters and mink predation. digest. It was observed that the cormorants often stood on Coots feed on open water, and while feeding they are objects that protruded from the water. Juvenile cormorants very vocal and spend a lot of time displaying aggressive were seen perched on objects that were only just protruding behaviour. Aggressive interaction with conspecifics is an from the water ( personal observation). This social exclusion energy-reducing activity; it reduces time vigilance in quarry may increase predation risk in juvenile cormorants. ......................................................................................................................................................................................................................................... 146 Bioscience Horizons † Volume 1 † Number 2 † June 2008 Research article ......................................................................................................................................................................................................................................... 3. Chanin P (1981) The diet of otter and it relations with the feral mink in two Some birds associate otters as a threat to themselves and their areas of Southwest England. Acta Theriol 26: 83–95. nests. Black-headed gull were seen displaying mobbing beha- 4. Webb JB (1975) Food of the otter (Lutra lutra L.) on the Somerset Levels. viour towards an otter on 12 August 2006. While collecting J Zool 177: 486–489. spraints, three black-headed gulls were observed flying up and 5. Jenkins D, Harper RJ (1980) Ecology of otters in Northern Scotland: analysis down a bed of phragmites.Anotter emergedfrom the of otter (Lutra lutra) and mink (Mustela vision) faeces from Deeside N.E. reedbed and entered the water, the gulls continued to mob the Scotland 1977–1978. J Appl Ecol 49: 737–754. otter. Clode et al. investigated the mobbing behaviour of 6. Mason CF, MacDonald SM (1986) Otters Ecology and Conservation. black-headed gull and terns (Sterna spp.). Their findings Cambridge: Cambridge University Press. suggest that birds offer differing responses to predators. Adult 7. Lanszki J, Sallai Z (2006) Comparison of the feeding habits of Eurasian otters on a fast flowing river and its backwater habitats. Mamm Biol. 71: 346–366. birds hover over animals that are potential threat to them, 8. McMahon J, McCafferty DJ (2006) Distribution and diet of otters (Lutra lutra) which is classed as a fleeing response. Birds that feel their eggs in marine areas of Loch Lomond and The Trossachs National Park, Scotland, and young are threatened often offer an aggressive response to UK. Lutra 49: 29–36. the predator. They will attempt to drive the predator away 9. Chown D (2005) Shapwick Heath NNR: Waterfowl Populations and Breeding from the young, by repeatedly diving towards the predator. Success in 2004, and Comparison with 2002 and 2003. Peterborough: This aggressive response to potential predators carries the English Nature. risks of death or injury to the mobbing adult birds, so much 10. Jenkins D (1980) Ecology of otters in Northern Scotland: I. Otter (Lutra lutra) so that the black-headed gulls chose the fleeing type of breeding and dispersion in Mid-Deeside, Aberdeenshire in 1974–1979. J Anim Ecol 49: 713–35. mobbing. Mobbing behaviour is learned through experience 11. Jenkins D, Burrows GO (1980) Ecology of otters in Northern Scotland III. The possibly implying that otters have attempted to predate upon use of faeces as indicators of otter (Lutra lutra) density and distribution. J black-headed gulls and that otters are now perceived as a Appl Ecol 49: 755–774. threat by black-headed gulls on Shapwick Heath. Though 12. Bas N, Jenkins D, Rothery P (1984) Ecology of otters in Northern Scotland: II. The there was no evidence in the spraints of gulls being taken by distribution of otter (Lutra lutra) faeces in relation to bank side vegetation on otters, the adaptation of anti-predator by black-headed gulls the River Dee in summer 1981. J Appl Ecol 21: 507–513. suggests that they are predated on. 13. Day MG (1966) Identification of hair and feather remains in the gut and Otters are not perceived as threats by all birds. Grey faeces of stoats and weasels. J Zool 148: 201–207. herons and mute swans took no notice of otters in water 14. Conroy JWH, Watt J, Webb JB et al. (2005) A Guide to the Identification of the Prey Remains in Otter Spraint. London: The Mammal Society. near to them, which may be due to the bird’s larger size. 15. Prast W, Shamoun J (2001) Bird Remain Identification System (BRIS). Amsterdam: Grey herons have been reported as prey items of otter in 23, 24 Expert Centre for Taxonomic Identification, University of Amsterdam. Spain and suggested that the otters are killing the 16. Elmberg J, Sjo¨ berg K, Nummi P et al. (2004) Patterns of lake acidity and heron as an act of super-predation rather than killing the waterfowl communities. Hydrobiologia 279: 201–206. herons to consume them. 17. Knights B (2003) A review of the possible impacts of long-term oceanic and Therefore, it may be concluded from this study that the climate changes and fishing mortality on recruitment of anguillid eels of the otters on Shapwick Heath demonstrate a larger proportion Northern Hemisphere. Sci Total Environ 310: 237–244. of birds in otters’ diet than that from previous studies. 18. Ferraras P, Macdonald DW (1999) The impact of mink (Mustela vision) on Whether this is due to the particularly high local availability water birds in upper Thames. J Appl Ecol 36: 701–708. of birds as a food source at Shapwick Heath and whether this 19. Irwin J, O’Halloran O (1997) The wintering behavior of coot Fulica atra at Cork Lough, South West Ireland. Biology and the Environment. Proc R Irish level of predation impacts significantly on the overall bird Symp 97B: 157–162. population levels would require further research. 20. Macdonald DW, Harrington LA (2003) The American Mink; the triumph and tragedy of adaptation out context. NZ J Zool 30: 421–441. Acknowledgements 21. Gre´millet D, Wilson RP (1999) A life in the fast lane: energetics and foraging strategies of the great cormorant. Behav Ecol 5: 516–524. The author would like to thank James Williams of the 22. Clode D, Birks JDS, Macdonald DW (2000) The influence of risk and vulner- Somerset Otter Group for advice concerning otter spraints. ability on predator mobbing by terns (Sterna spp.) and gulls (Larus spp.). J Zool 252: 53–59. 23. Ruiz-Olmo J, Marsol R (2002) New information on the predation of fish References eating birds by the Eurasian otter (Lutra lutra) IUCN Otter spec. Group Bull 19: 103–106. 1. Carrs DN (1995) The foraging behaviour and feeding ecology of the otter (Lutra lutra): a selective review. Hystrix 7: 179–194. 24. Ruiz-Olmo J, Marsol R, Asensio V (2003) More on predation of fish eating birds by Eurasian otters (Lutra lutra) in captivity. IUCN Otter spec. Group 2. Kruuk H (2006) Otters, Ecology, Behaviour and Conservation. Oxford: Oxford Bull 20: 73. University Press. ........................................................................................................................................................................................................................................ Submitted on 1 October 2007; accepted on 28 January 2008; advance access publication 23 April 2008 .........................................................................................................................................................................................................................................

Journal

Bioscience Horizons – Oxford University Press

Published: Jun 23, 2008

Keywords: Key words Lutra lutra Shapwick Heath diet ralliformes spraint feather

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