Egg coloration and selection for crypsis in open‐nesting blackbirds

Abstract

High variation in egg coloration among birds has traditionally been explained as adaptation for camouflage. We tested this hypothesis by conducting reciprocal clutch exchanges (n=301) among Brewer's blackbirds Euphagous cyanocephalus, red‐winged blackbirds Agelaius phoeniceus, and yellow‐headed blackbirds Xanthocephalus xanthocephalus. We predicted that clutches placed against their natural nest backgrounds would have higher survival rates than heterospecific clutches. Intraspecific clutch exchanges were used as a control. Clutch survival was monitored for a 9‐d period at all nests, during which time incubation rhythm and nest defense were quantified. Intraspecific clutch exchanges did not influence incubation or nest defense. For two of the species, intraspecific clutch exchanges did not influence clutch survival; in red‐winged blackbirds, however, intraspecifically exchanged clutches had somewhat depressed survival curves relative to control clutches (P=0.08). The effect of interspecific clutch exchanges differed by host species. In Brewer's nests, eggs of the yellow‐headed blackbird had lower survival than Brewer's eggs (P=0.02), but survival of red‐winged blackbird eggs did not differ from Brewer's eggs (P=0.50). In nests of red‐winged blackbirds, all three clutch types had approximately equal survival. In yellow‐headed blackbird nests, eggs of the red‐winged blackbird had lower survival than yellow‐headed blackbird eggs (P=0.06), and survival of Brewer's eggs did not differ from yellow‐headed blackbird eggs (P=0.31). These findings support a role for egg coloration as camouflage in two of the three species studied.