Effects of the introduced parasite Philornis downsi
on nestling growth and mortality in the medium
ground ﬁnch (Geospiza fortis)
Sarah K. Huber*
Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts, 319 Morrill Science Center,
Amherst, MA 01003, USA
Received 27 May 2007
Received in revised form
13 November 2007
Accepted 25 November 2007
Available online 31 January 2008
Invasive species have the potential to detrimentally affect native ecosystems by out com-
peting or directly preying upon native organisms, and have been implicated in the extinc-
tion of endemic populations. One potentially devastating introduced species in the
pagos Islands is the parasitic ﬂy Philornis downsi. As larvae, P. downsi parasitize nestling
birds and have been associated with high nestling mortality and reduced growth rates.
Here I document nestling growth and mortality in a bimodal population of the medium
ground ﬁnch, Geospiza fortis. Observations were conducted over three years, and under var-
iable ecological conditions. Annual parasite prevalence in nests ranged from 64% to 98%,
and nestling mortality in nests with parasites ranged from 16% to 37%. Parasite load and
parasite load per nestling follow a skewed distribution with many nests having relatively
few parasites, and few nest having many. Parasite load, however, was not correlated with
onset of breeding, clutch size, the number of nestlings, nestling survival or ﬂedgling suc-
cess. Parasite load per nestling, on the other hand, was correlated with clutch initiation
date and the proportion of nestlings that died in parasitized nests. Neither nestling size
nor growth rate differed between parasitized and unparasitized nests. In addition, male
and female beak morphology was not correlated with parasite load, breeding variables or
nestling survival. Thus, while overall mortality due to parasitism is high, ecological condi-
tions and possible host defenses may potentially counter some of the detrimental affects of
P. downsi on nestling size and growth. These results taken together suggest that parasitism
of P. downsi larvae on nestling G. fortis has the potential to lead to large population declines.
Ó 2007 Elsevier Ltd. All rights reserved.
The ecological and evolutionary impacts of invasive species
on native organisms has been a major emphasis of recent re-
search in conservation biology (Sakai et al., 2001; Mooney and
Cleland, 2001; Snyder and Evans, 2006; Strayer et al., 2006;
Weinig et al., 2007). Invasive species often out-compete or
prey on native species, and have been implicated in extinc-
tions of native organisms (Warner, 1968; Van Riper et al.,
1986, 2002; Vitousek et al., 1987; Mack et al., 2000). Island
0006-3207/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
* Present address: Department of Biology, University of Utah, 257 South 1400 East Salt Lake City, UT 84112, USA. Tel.: +1 413 695 7069.
E-mail address: email@example.com.
141 (2008) 601– 609
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/biocon