title: The role of egg-nest contrast in the rejection of brood parasitic eggs.
abstract: Hosts of avian brood parasites can avoid the reproductive costs of raising genetically unrelated offspring by rejecting parasitic eggs. The perceptual cues and controls mediating parasitic egg discrimination and ejection are well studied: hosts are thought to use differences in egg color, brightness, maculation, size and shape to discriminate between their own and foreign eggs. Most theories of brood parasitism implicitly assume that the primary criteria to which hosts attend when discriminating eggs are differences between the eggs themselves. However, this assumption is confounded by the degree to which chromatic and achromatic characteristics of the nest lining co-vary with egg coloration, so that egg-nest contrast per se might be the recognition cue driving parasitic egg detection. Here, we systematically tested whether and how egg-nest contrast itself contributes to foreign egg discrimination. In an artificial parasitism experiment, we independently manipulated egg color and nest lining color of the egg-ejector American robin (Turdus migratorius), a host of the obligate brood parasitic brown-headed cowbird (Molothrus ater). We hypothesized that the degree of contrast between foreign eggs and the nest background would affect host egg rejection behavior. We predicted that experimentally decreasing egg-nest chromatic and achromatic contrast (i.e. rendering parasitic eggs more cryptic against the nest lining) would decrease rejection rates, while increasing egg-nest contrast would increase rejection rates. In contrast to our predictions, egg-nest contrast was not a significant predictor of egg ejection patterns. Instead, egg color significantly predicted responses to parasitism. We conclude that egg-egg differences are the primary drivers of egg rejection in this system. Future studies should test for the effects of egg-nest contrast per se in predicting parasitic egg recognition in other host-parasite systems, including those hosts building enclosed nests and those parasites laying cryptic eggs, as an alternative to hypothesized effects of egg-egg contrast.
authors.affiliation: Biopsychology and Behavioral Neuroscience Subprogram in Psychology, The Graduate Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA Department of Psychology, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, USA Social and Behavioral Sciences Division, Bloomfield College, Bloomfield, NJ 07003, USA Zachary_aidala@bloomfield.edu.
authors.forename: Zachary
authors.initials: Z
authors.lastname: Aidala
authors.affiliation: Ecology, Evolutionary Biology, and Behavior Subprogram in Biology, The Graduate Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA Department of Biology - Program in Ecology, Evolution, and Conservation Biology, University of Nevada - Reno, Reno, NV 89503, USA.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Croston
authors.affiliation: Department of Psychology, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, USA.
authors.forename: Jessica
authors.initials: J
authors.lastname: Schwartz
authors.affiliation: Department of Psychology, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, USA.
authors.forename: Lainga
authors.initials: L
authors.lastname: Tong
authors.affiliation: Biopsychology and Behavioral Neuroscience Subprogram in Psychology, The Graduate Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA Department of Psychology, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, USA Ecology, Evolutionary Biology, and Behavior Subprogram in Biology, The Graduate Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Hauber
mesh_terms: D000818:Animals; D003116:Color; D003463:Cues; D009425:Nesting Behavior; D010063:Ovum; D046109:Passeriformes / Q000502:physiology*; D020308:Songbirds / Q000502:physiology
keywords: Brood parasitism; Egg rejection; Visual ecology; Visual modeling