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 eggegg 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.

Experimental shifts in egg–nest contrasts do not alter egg rejection responses in an avian host–brood parasite system

“…1 ). However, as also seen in the American robin (Aidala et al 2015 ), egg-egg contrast is negatively correlated with egg-nest contrasts between experimental eggs and the natural nest lining's coloration (Fig. 1 ).…”

“…To date, only one study has directly assessed whether avian perceivable egg-nest contrast per se is a proximate cause of foreign egg rejection by hosts of brood parasitic birds: Aidala et al ( 2015 ) found no causal relationship between egg-nest contrast and the rejection of red, blue (host mimetic), and beige (parasite mimetic) plaster eggs by American robins Turdus migratorius, an egg rejecter host of the generalist and non-mimetic brood parasitic brown-headed cowbirds Molothrus ater in North America (Croston and Hauber 2015b ). Parallel studies reported here have focused on the great reed warbler Acrocephalus arundinaceus, a commonly parasitized host of a highly mimetic host race of the common cuckoo Cuculus canorus in Hungary (Moskár) and elsewhere (Drobniak et al 2014 ).…”

Which egg features predict egg rejection responses in American robins? Replicating Rothstein's (1982) study

“…If the nest was deemed abandoned (or deserted), eggs in the nest hatched, or the nest was depredated during the experimental period, the experimental trial was ended and excluded from the analyses. For a more detailed explanation of our experimental parasitism procedures on robins using plaster of Paris model eggs, see Croston and Hauber () and Aidala, Croston, Schwartz, Tong, and Hauber (). Critically, these studies found no effect of repeated parasitism and nesting stage (laying vs. incubation) on egg rejection rates by American robins.…”

Grey Gerygone hosts are not egg rejecters, but Shining Bronze-Cuckoos lay cryptic eggs

“…For example, if hosts are able to recognize and reject cuckoo eggs, cuckoos may evolve eggs that mimic those of their hosts (Soler 2014). However, not all hosts of virulent cuckoos evolve egg rejection behavior (Kilner and Langmore 2011); some hosts seem to ignore a cuckoo's egg even when it is conspicuous against the background of their nest (Aidala et al 2015) or appears very different from their own eggs (Stoddard and Stevens 2011). Similarly, not all cuckoos have evolved mimetic eggs.…”