Meeting Abstract
105.2 Sunday, Jan. 6 Spectral tuning and foreign egg rejection in American robins (Turdus migratorius) CROSTON, R.*; HAUBER, M.E.; CUNY Graduate Center; CUNY Hunter College, CUNY Graduate Center RCroston@gc.cuny.edu
By laying their eggs in the nests of other birds, avian brood parasites impose the cost of rearing their young upon the hosts. Recognition and rejection of foreign eggs are the primary and most effective host defenses against costly brood parasitism. Yet, hosts of parasitic brown-headed cowbirds (Molothrus ater) challenge co-evolutionary theory because most hosts accept parasitic eggs despite their drastically different appearance from the hosts’ own. American robins (Turdus migratorius) are one of few cowbird hosts to reject foreign eggs. Previous research yielded equivocal evidence whether egg rejection by robins evolved specifically in response to cowbird parasitism, or is based on recognition of own eggs and not specific to cowbird eggs. Our research employed avian visual perceptual modeling and behavioral experimentation to investigate mechanisms driving parasitic egg rejection in robins. We modeled effects of overall chromatic difference as JNDs (just noticeable differences) on rejection rates in response model eggs with artificial colors spanning the entire avian spectral sensitivity range. We then modeled effects of differences in quantum photoreceptor catches between natural and model eggs to determine which photoreceptor inputs best predict the rejection responses. The model best predicting rejection rates contained values from all photoreceptor types in the avian visual system, but JND values were not significant. Experimental eggs mimicking cowbird egg ground color were rejected in all experimental trials, but these differed little in JND value from both real and model robin mimetic eggs, which were typically accepted. We propose a nested rejection criterion where foreign egg rejection is driven primarily by differences across most regions of the avian visual spectrum, but beige eggs (as laid by parasitic cowbirds) are also always rejected.
