Most altricial birds remove their nestlings’ feces from the nest, but the evolutionary forces driving this behavior are poorly understood. A possible adaptive explanation for this could be that birds avoid the attraction of nest predators to their nests due to the visual or olfactory cues produced by feces (nest predation hypothesis). This hypothesis has received contrasting support indicating that additional experimental studies are needed, particularly with respect to the visual component of fecal sacs. To test this hypothesis, we conducted an experiment manipulating the presence of fecal sacs on inactive Woodlark (Lullula arborea) nests. This ground nester has highly cryptic nests that are mainly depredated by visually oriented nest predators (i.e., corvids) in our study population, making it an excellent system to test for the nest predation hypothesis. Our results showed that the presence of fecal sacs in the nest does not seem to be an important factor explaining nest predation. Interestingly, the effect of nest concealment, the most important factor explaining nest predation in Woodlark nests, depended on whether the nest was depredated the previous year or not, supporting the importance of using different nesting sites between years. Our findings indicate that this important nest sanitation behavior is not likely motivated by nest predation and highlight the need to explore alternative selective pressures in this context. Keywords Nest sanitation · Visual cues · Nest predation hypothesis · Lullula arborea · Nest concealment · Woodlark Zusammenfassung Kotballen führen bei Bodenbrütern nicht zu mehr Nestraub. Bei den meisten Nesthocker-Arten entfernen die Altvögel die Ausscheidungen ihrer Jungen aus dem Nest, aber die evolu- tionären Kräfte hinter diesem Verhalten sind noch weitgehend unbekannt. Eine mögliche Erklärung des Anpassungsvor- teils dieser Verhaltensweise könnte sein, dass die Vögel von den Ausscheidungen ausgehende optische oder olfaktorische Reize, die die Aufmerksamkeit von Nesträubern auf ihr Nest lenken könnten, beseitigen (Nesträuber-Hypothese). Diese Hypothese findet allerdings nur gemischte Unterstützung, was zeigt, dass hier weitere experimentelle Untersuchungen vonnöten sind, vor allem hinsichtlich des optischen Aspekts der Kotballen. Um diese Hypothese zu prüfen, führten wir ein Experiment durch, in dem wir bei inaktiven Nestern der Heidelerche (Lullula arborea) das Vorhandensein von Kotballen manipulierten. Diese Bodenbrüter bauen sehr versteckte Nester, die in unserer Versuchspopulation überwiegend von sich optisch orientierenden Nesträubern (z. B. Rabenvögeln) geplündert wurden, und somit ein sehr gutes Test-System für die Nesträuber-Hypothese darstellten. Unsere Ergebnisse zeigten, dass das Vorhandensein von Kotballen im Nest kein wichtiger Faktor bei der Nesträuberei ist. Interessanterweise hing die Bedeutung, wie gut das Nest versteckt ist, der Nestraub am besten erklärende Faktor, davon ab, ob das Nest im Vorjahr ausgeraubt wurde oder nicht. Dies unterstreicht, wie wichtig es ist, in unterschiedlichen Jahren unterschiedliche Nistplätze zu nutzen. Unsere Ergebnisse weisen darauf hin, dass dieses wichtige Nestsäuberungsverhalten nicht unbedingt mit Nesträuberei erklärt werden kann, und unterstreichen die Notwendigkeit, nach anderen Selektionsmechanismen dafür zu suchen. Communicated by F. Bairlein. Introduction * Juan Diego Ibáñez-Álamo firstname.lastname@example.org Nest sanitation is an important component of parental care, widely present in birds but still poorly understood (Ibáñez- Groningen Institute for Evolutionary Life Sciences, Álamo et al. 2017). The removal of nestling excrements, University of Groningen, 9700 CC Groningen, The Netherlands Vol.:(0123456789) 1 3 986 Journal of Ornithology (2018) 159:985–990 probably one of the main nest sanitation activities car- In order to test the nest predation hypothesis, we experi- ried out by altricial birds (Guigueno and Sealy 2012), has mentally manipulated the presence of feces in inactive received increasing attention in the last years with a spe- Woodlark (Lullula arborea) nests. This is an ideal species in cial focus on experimental studies exploring the adaptive which to test the nest predation hypothesis, and particularly significance of such behavior (e.g., Ibáñez-Álamo et al. the effect of visual cues of fecal sacs, as it suffers from an 2013, 2014a; Quan et al. 2015). In most of the species, elevated nest predation pressure by visually oriented preda- the nestlings’ feces are encapsulated in a mucous covering tors (Praus et al. 2014) and has evolved several adaptations (the fecal sac) (e.g., Herrick 1900; Pycraft 1909; Thomp- to avoid it, including highly cryptic nests (Donald 2017). son 1934) which facilitates their manipulation by parents In addition, adult Woodlarks remove all their nestling feces (Ibáñez-Álamo et al. 2017). Furthermore, the removal of (Blair and Tucker 1941), indicating that their presence in the the nestlings’ feces has been suggested to drive the evo- nest might be a risk factor potentially increasing their nest lution of fecal sacs (Ibáñez-Álamo et al. 2017), which predation risk. We predicted that nests with fecal sacs would emphasizes the importance of nest sanitation for life his- be significantly more preyed upon than those without them. tory traits in altricial birds. One of the main hypotheses proposed to explain the removal of nestling feces from the nests of altricial birds Methods is the nest predation hypothesis, which states that the pres- ence of feces in the nest will attract predators (Herrick The study was conducted in Aekingerzand, within the 1900; Weatherhead 1984; Petit and Petit 1987). Nest pre- Drents-Friese World National Park, in the north of the dation is a key factor modulating parental care behaviors in Netherlands (52°56′N, 6°17′E) during April–June of 2016. birds such as incubation or food delivery to nestlings (e.g., This is a large area of heather, grass, moss, and bushes sur- Ghalambor et al. 2013; Morosinotto et al. 2013; Hua et al. rounded by coniferous forest where Woodlarks breed from 2014; reviewed in Martin and Briskie 2009; Ibáñez-Álamo March to July (Hegemann 2012). The main predators of et al. 2015). According to this hypothesis, the presence Woodlark nests in the study site are visually oriented corvids of feces in active nests could attract nest predators due (Carrion Crows Corvus corone and Eurasian Jays Garrulus to visual or olfactory cues (Ibáñez-Álamo et al. 2014a). glandarius) (Praus et al. 2014). Visual cues would be due to the white part of the fecal sac We searched for active Woodlark nests in our study area and would likely be easily detected by visually oriented from the beginning of the breeding season. Adults carry- predators (i.e., birds), whereas the olfactory cues would ing nest material or food in the beak were found by direct likely favor the attraction of olfactory oriented predators observation and followed to the nest. All nests found were (i.e., mammals). However, several studies showed that dif- visited regularly (every 3 days) until hatching. Once a nest ferent visual cues in the nest do not increase nest predation was depredated during the incubation stage, it was collected significantly, as visual predators tend to detect nests rather and stored in a plastic bag in the field station until its utili- than their contents (Götmark 1992; Weidinger 2001). To zation. A nest was considered as preyed upon when no egg our knowledge, there are only two published experiments remains were left in the nest. We only collected nests depre- exploring the nest predation hypothesis and they provide dated during the incubation stage (and not during the nest- contrasting results. In the first experimental study on nest ling stage) to avoid potential confounding effects of having sanitation, Petit et al. (1989) demonstrated that the pres- some nests with the scent of nestlings and others without it. ence of feces close to artificial nests increased their pre - Using this procedure, we collected 60 depredated Woodlark dation. However, the authors suggested that their results nests in total. were difficult to interpret because of the artificial nature We placed these depredated nests on known Woodlark of their experiment, which involved using non-specific nesting locations from the previous year (2015) that had artificial ground nests and chicken feces covered with a been marked with a Global Positioning System device, there- mixture of water and flour. More recently, another experi- fore using real sites selected by Woodlarks. Information on mental study using real nestling feces and active Common whether nests were preyed upon or successful in these loca- Blackbird (Turdus merula) nests, found no support for the tions the previous year was also available. All inactive nests attraction of nest predators due to feces (Ibáñez-Álamo were baited with two Japanese Quail eggs (Coturnix japon- et al. 2014b). This study, however, tested the olfactory ica) because they are also cryptic and laid in cryptic nests on component of the feces exclusively. Therefore, additional the ground, therefore minimizing the influence of additional studies also considering the visual component of nestling visual cues in our experimental setup. Furthermore, two feces are required to test whether fecal sacs really attract Japanese Quail eggs (mean volume = 49.9 cm ) also offered nest predators. a similar energetic reward for nest predators as an entire Woodlark clutch of four eggs [mean volume = 37.2 cm ; 1 3 Journal of Ornithology (2018) 159:985–990 987 mean clutch size in the population 4.02 eggs (Horrocks et al. We tested the effect of our treatment on the daily survival 2014)]. Once the nest was placed on its location, we per- rate of our inactive Woodlark nests by using a model selec- formed the following treatments following a similar experi- tion based on the second-order Akaike information criterion mental design as that used by Ibáñez-Álamo et al. (2014a). (AICc) (Burnham and Anderson 2002) and the packages The first treatment comprised an experimental group to R Mark (version 2.2.4; Laake 2013) and lubridate (ver- which we added two fecal sacs at the rim of the nest at every sion 1.7.1.; Grolemund and Wickham 2011). R Mark is an visit in order to mimic the natural accumulation of feces in interface to run nest survival models in the program MARK an unattended nest. In the second treatment, comprising the (White and Burnham 1999). As additional predictors, we manipulation control group, we added a similar weight of also included in the model selection the visibility index, the mud (mean ± SE 2.37 ± 0.26 g; obtained from the vicinity previous-year status [depredated (1) or not (0)], temporal of the nest) as the excrements added to the experimental trial (1–4) and all two-way interactions. We used the package group previously described (mean ± SE 2.38 ± 0.18 g; linear MuMin (version 1.40; Bartoń 2017) to calculate the model- model, F = 0.0002; df = 1; p = 0.99). The main visual dif- averaged coefficients and relative importance of each predic- ference between feces and mud was the conspicuous white tor of those models with a weight > 1%. The analyses were part typical of fecal sacs. The third treatment comprised a done using R software (version 3.4.2; R Core Team 2017). control group that was visited in a similar way but to which nothing was added. Blackbird fecal sacs were used for the experimental nests Results due to the low availability of Woodlark nests with chicks from which to collect Woodlark feces. Blackbird fecal sacs Our results indicate that the best model explaining the ee ff ct do not attract predators to nests due to their olfactory cues of the presence of fecal sacs is that containing the interaction (Ibáñez-Álamo et al. 2014a) and offer similar visual cues of previous-year status by visibility index (Table 1). The as Woodlark fecal sacs (personal observation). Blackbird model that includes the same interaction in addition to the nestlings easily defecate when handled (Ibáñez-Álamo et al. temporal trial, as well as that with the latter as the only pre- 2014b). Once collected, fecal sacs were preserved cold dictor are also considered equally parsimonious (ΔAICc <2) (4 °C) in a small container with water and added to the nests (Burnham and Anderson 2002) though they have a smaller within the following 24 h. This method allowed us to mimic weight (Table 1). The interaction of previous-year status by freshly produced fecal sacs keeping intact their mucous visibility index is also among the most important predictors, covering and water content. This is particularly important in addition to the other individual variables included in the as alterations in these factors could potentially affect their most parsimonious models (Table 2). The model-averaged detectability. All nests were visited every 2 days during coefficients indicated that the increased predation associ - a 9-day period or until they were depredated. This 9-day ated with a low nest concealment (high visibility index) only period is the mean duration of the nestling period for Wood- applies to those sites whose Woodlark nests were depre- larks in our population (Praus et al. 2014). We considered a dated the previous year. On the contrary, we found very little nest depredated if the eggs were either broken or missing. support that our experimental treatment on nest predation Finally, as nest predation may vary during the breeding sea- affected the daily survival rate of the inactive Woodlark nests son (Picman and Schriml 1994; Weidinger 2001), to avoid (Tables 1, 2), suggesting that fecal sacs did not increase the a temporal bias in our findings we distributed our inactive probability of nest predation (Table 3). nests in four temporal groups (starting 25 April, 4 May, 20 May and 15 June). Each temporal group consisted of 15 nests, five per treatment. Discussion We also calculated a visibility index for each nest using a categorical variant of a method previously published (Bayne Our experiment does not support the nest predation hypoth- and Hobson 1997). The same observer (E. R.) graded (0–2) esis and suggests that nestlings’ feces do not attract more the visibility of each nest to the human eye from a distance predators to the nest, at least in the Woodlark. Our results, of 2 m in each of the four cardinal directions. The sum of therefore, are in agreement with those obtained by another the values obtained in each cardinal point established the recent experimental study testing this hypothesis (Ibáñez- visibility index of the nest (range 1–6 in our dataset). The Álamo et al. 2014a). That investigation, using a similar visual score was calculated as: 0, when neither the nest nor experimental design did not find evidence to support that the eggs could be seen; 1, when part of the nest or the eggs the olfactory component of fecal sacs increased the risk of could be seen but not completely; 2, when the nest and the predation of blackbird nests. Here, we found a similar result eggs could be seen completely. in a more visually oriented context indicating that the white part of fecal sacs does not seem to promote the detectability 1 3 988 Journal of Ornithology (2018) 159:985–990 Table 1 Model selection results Model K AICc ΔAICc Weight from the R Mark analysis of daily survival rate indicating Previous year status × visibility 4 162.05 0.0 0.24 the number of parameters Previous year status × visibility + temporal trial 5 162.27 0.21 0.21 considered in the model (K), Temporal trial 2 163.81 1.76 0.10 Akaike information criteria value corrected for small sample Visibility 2 164.22 2.16 0.08 sizes (AICc), the difference in Constant DSR 1 164.58 2.53 0.07 AICc values between a given Previous year status + visibility 3 164.59 2.54 0.07 model and that with the lowest Previous year status × visibility + temporal trial + treatment 7 164.84 2.78 0.06 AICc value (ΔAICc), and the Akaike weights for each model Visibility × Temporal trial 4 165.25 3.20 0.05 Previous year status 2 165.63 3.58 0.04 Previous year status × temporal trial 4 166.91 4.85 0.02 Only the ten most supported models are included in the table DSR Daily survival rate Table 2 Model-averaged Predictor Estimate SE 2.5% CI 97.5% CI Importance coefficients (± SE), confidence intervals (CIs) and relative Intercept 0.375 1.571 − 2.707 3.457 importance for each predictor Visibility 0.609 0.549 − 0.159 1.768 0.76 Previous year status (1) 1.633 1.697 − 0.584 5.440 0.67 Previous year status (1) × visibility − 0.494 0.585 − 1.883 − 0.021 0.52 Temporal trial 0.008 0.013 − 0.012 0.046 0.46 Treatment (experimental) − 0.058 0.236 − 1.523 0.492 0.11 Treatment (manipulation control) − 0.025 0.183 − 1.217 0.764 Temporal trial × visibility − 0.001 0.003 − 0.025 0.011 0.05 Temporal trial × previous-year status (1) − 0.001 0.004 − 0.055 0.036 0.02 Table 3 Daily nest survival (± SE) and overall survival rate (9-day study that tested the nest predation hypothesis. The authors period) for each of the three treatments in our experiment obtained of that study already acknowledged that their findings could from the minimum model (~ treatment) be an artifact due to the artificial methodology used includ- Treatment DSR SE Overall ing unspecific artificial nests (a Quail egg directly placed survival on the ground) and artificial nestling feces. The use of rate artificial nests in nest predation studies has been recom - Experimental 0.894 0.024 0.366 mended only to test specific hypotheses and once the nest predator community of the focal species has been identified Manipulation control 0.917 0.030 0.459 Control 0.926 0.025 0.500 (Ibáñez-Álamo et al. 2015), like in our case, as otherwise they may lead to inaccurate conclusions given the important Sample size for each treatment is 20 differences that they may have with natural nests (Major and Kendal 1996; Davidson and Bollinger 2000). Another of nests. This finding fits with the idea that nest contents are important difference between Petit et al.’s (1989) study and not the key factor used by nest predators in order to look ours that might explain the opposite results is their use of for nests (Götmark 1992; Weidinger 2001). Our results are adult chicken feces mixed with a solution of flour and water also in agreement with other observational studies showing in contrast to our use of fresh and natural fecal sacs, which that nest predation does not seem to influence other aspects might have attracted in their case an unnatural community of feces removal like the direction of feces transportation of nest predators (i.e., adult birds or chicken predators). In (Weatherhead 1984; Petit and Petit 1987), or additional fact, they indicated that even though the visual component experimental evidence indicating that it is not an important of their artificial nestling feces was similar to real fecal sacs, factor explaining the ingestion of fecal sacs (Ibáñez-Álamo the olfactory component seemed to be completely differ - et al. 2013). ent. This in addition to the fact that they estimated that the On the other hand, our findings contrast markedly with majority of nest predators were (olfactory oriented) mam- those obtained by Petit et al. (1989) in another experimental mals led them to suggest that their effect will be attributed 1 3 Journal of Ornithology (2018) 159:985–990 989 Acknowledgments We are very thankful to Steff Waasdorp, Frederique to the odor of feces, and therefore, not contradictory to our Derks and Henri Zomer for their help during the fieldwork and spe- results. Alternatively, the different results in the two studies cially in nest searching. Joseph Mwangi and Maaike Versteegh made could be explained by the effect of the mucous covering of very useful suggestions for the statistical analyses. We would like to fecal sacs, only present in our study. It might be possible that thank two anonymous reviewers who made very useful suggestions which improved the manuscript. This project was financed by NWO the mucous covering (not found in adult feces) reduces the Vidi Grant 864.10.012 to B. I. T. We thank Staatsbosbeheer for permis- detectability of nestling feces by predators, either by reduc- sion to work in the Aekingerzand. ing their visual or olfactory cues. However, to our knowl- edge, this possibility has not been investigated so far, and Compliance with ethical standards in fact, the adaptive function(s) of this trait is not clear yet (Ibáñez-Álamo et al. 2014b, 2017). Ethics statement The authors declare that the experiments carried out Our results also indicate that the location of the nest in this work comply with current Dutch and International laws. might be an important factor determining predation. Some studies on ground nesters suggest that nesting at the same Conflict of interest The authors declare that they have no conflict of site increases the probability of being preyed upon (Martin interest. et al. 2000; Yahner and Mahan 1996). Initially, our results Open Access This article is distributed under the terms of the Crea- seem to be contradictory to these latter studies as we found tive Commons Attribution 4.0 International License (http://creat iveco a positive effect of previous-year status on nest survival. mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- However, this effect was mediated by nest concealment tion, and reproduction in any medium, provided you give appropriate in Woodlarks, indicating a dominant role of detectability credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. rather than long-term memory by predators in explaining the between-year repeatability of predation events. Nest con- cealment has been proposed to have an important effect in other ground nesters too (e.g., Gregg et al. 1994). 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Journal of Ornithology – Springer Journals
Published: May 29, 2018
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