Behavioral Defenses against Avian Brood Parasitism in Sympatric and Allopatric Host Populations

Abstract

The brown-headed cowbird (Molothrus ater) is a widespread, obligate brood parasite of North American passerine birds. In southern Manitoba, where hosts are sympatric with cowbirds, American robins (Turdus migratorius) ejected parasitic eggs from all experimentally parasitized clutches (N = 25) and no eggs were accepted for more than four days. In contrast, robins in northern Manitoba, an area where cowbirds do not breed, accepted parasitic eggs in 33% of nests (N = 18) for at least five days. Acceptance of experimental cowbird eggs by a second host, the yellow warbler (Dendroica petechia), was similar in allopatric (100% of 20 nests) and sympatric (88.6% of 35 nests) populations, but models of a female cowbird elicited greater nest defense by warblers in the area of sympatry. Neither host rejected eggs of conspecifics, thus, rejection of cowbird eggs was not an epiphenomenon of conspecific brood parasitism. These results support the hypothesis that recognition of cowbirds and their eggs evolved as adaptations to counter cowbird parasitism and not some other selection pressure. The expression of anti-parasite defenses by some individuals within allopatric populations further suggests these traits may be controlled genetically but persist in such areas either through the continued introgression of rejecter genes from sympatric populations or because of the low cost of rejection behavior when parasitism is absent or rare.

Geographic variation in egg ejection rate by great tits across 2 continents

“…However, Møller and Soler (2012) recently reviewed the literature on consistency in host responses to brood parasites and found large intraspecific variation in resistance in several host species (Cruz and Wiley 1989;Nakamura 1990;Soler 1990;Soler and Møller 1990;Briskie et al 1992;Lindholm 1999;Soler, Martínez, et al 1999;Moskát et al 2002;Martín-Gálvez et al 2006Moskát et al 2008;Stokke et al 2008;Avilés et al 2011;Soler et al 2011;Langmore et al 2012). These studies indicate that speciesspecific consistency in resistance is far from the rule.…”

The role of egg-nest contrast in the rejection of brood parasitic eggs

“…Robins are a suitable study species in that they are one of fewer than 30 documented cowbird host species to eject cowbird eggs at rates above 75% (Briskie et al, 1992;Peer and Sealy, 2004), allowing for the testing of specific sensory hypotheses mediating egg rejection in this system. Previous work on this species pair showed that natural and model cowbird eggs are perceptually distinct from natural (conspecific) robin eggs: natural parasite eggs are rejected from 100% of experimental nests, whereas conspecific natural and model robin eggs are not rejected (Briskie et al, 1992;Croston and Hauber, 2014a;Rothstein, 1982;Fig. 1A).…”

Interspecific egg rejection as ecological collateral damage from selection driven by conspecific brood parasitism

“…With respect to either recognition or rejection, gene flow between parasitized and unparasitized populations could result in imperfect adaptations on a local level, and hence intermediate egg rejection rates (Briskie et al, 1992;Davies and Brooke, 1989). Alternatively, hosts may lag behind the parasites in the coevolutionary arms race such that the alleles for egg recognition or rejection have not completely swept through the population (Davies, 1999;Hosoi and Rothstein, 2000;Rothstein, 1990;Takasu, 1998).…”