Does contrast between eggshell ground and spot coloration affect egg rejection?

Abstract

Obligate avian brood parasitic species impose the costs of incubating foreign eggs and raising young upon their unrelated hosts. The most common host defence is the rejection of parasitic eggs from the nest. Both egg colours and spot patterns influence egg rejection decisions in many host species, yet no studies have explicitly examined the role of variation in spot coloration. We studied the American robin Turdus migratorius, a blue-green unspotted egg-laying host of the brown-headed cowbird Molothrus ater, a brood parasite that lays non-mimetic spotted eggs. We examined host responses to model eggs with variable spot coloration against a constant robin-mimetic ground colour to identify patterns of rejection associated with perceived contrast between spot and ground colours. By using avian visual modelling, we found that robins were more likely to reject eggs whose spots had greater chromatic (hue) but not achromatic (brightness) contrast. Therefore, egg rejection decision rules in the American robin may depend on the colour contrast between parasite eggshell spot and host ground coloration. Our study also suggests that egg recognition in relation to spot coloration, like ground colour recognition, is tuned to the natural variation of avian eggshell spot colours but not to unnatural spot colours.

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

“…() demonstrated robins’ rejection decisions are fine‐tuned to the gradient of natural egg colors, but robins ignore perceivable differences along artificial color gradients, a finding inconsistent with the internal “own egg versus foreign egg” template (or multiple threshold, sensu Hanley et al., ) hypothesis. Similarly, Dainson, Hauber, López, Grim, and Hanley () also found that robin egg rejection responses to egg spot coloration are likely tuned to a gradient of natural egg color patterns, where robins are more inclined to reject model eggs that have highly contrasting brown spots against a mimetic blue‐green robin egg background color.…”

“…Here, we confirmed that discrete, categorical differences in egg background color, maculation, and size are all important cues for foreign egg recognition in robins. However, recent experimental approaches have set a new standard, using model eggs, which vary continuously rather than discretely, along natural gradients of different background colors (Hanley et al., ), as well as maculation patterns and contrasts (Dainson et al., ), and sizes and shapes (Igic et al., ). Egg rejection studies performed with continuously varying model eggs, in combination with avian visual modeling (Avilés, ; Cassey et al., ; Spottiswoode & Stevens, ), allow for estimation of perceivable differences to the host species of interest for each model egg feature.…”

“…Thus, they may provide more robust, meaningful comparisons of the relative influence of each separate egg feature for a given host species’ egg recognition threshold than studies using model eggs which vary only discretely. Given the recent and rapid development of 3D‐printing to construct model eggs of differing shapes and sizes for use in artificial brood parasitism experiments (Igic et al., ), and our knowledge of the avian‐visible egg color, maculation, and pigment diversity (Hanley, Grim, Cassey, & Hauber, ), it is now certainly possible to design such a suite of experiments for other brood parasite–host species, like those already performed with robins (Dainson et al., ; Hanley et al., ).…”

“…Given the recent and rapid development of 3D-printing to construct model eggs of differing shapes and sizes for use in artificial brood parasitism experiments (Igic et al, 2015), and our knowledge of the avian-visible egg color, maculation, and pigment diversity (Hanley, Grim, Cassey, & Hauber, 2015), it is now certainly possible to design such a suite of experiments for other brood parasite-host species, like those already performed with robins (Dainson et al, 2017;Hanley et al, 2017).…”

Variation in multicomponent recognition cues alters egg rejection decisions: a test of the optimal acceptance threshold hypothesis

“…Colour is not the only important feature in egg discrimination decisions. The patterns generated through eggshell maculation, including spots and lines (hereafter, spots) are also important factors as they can generate unique egg signatures [20] that provide hosts with valuable information about egg ownership (own versus foreign) [21]. Hosts with spotted eggs are generally more likely to accept spotted foreign eggs, and hosts with unspotted eggs are generally more likely to Figure 1.…”