Egg discrimination along a gradient of natural variation in eggshell coloration

Abstract

Accurate recognition of salient cues is critical for adaptive responses, but the underlying sensory and cognitive processes are often poorly understood. For example, hosts of avian brood parasites have long been assumed to reject foreign eggs from their nests based on the total degree of dissimilarity in colour to their own eggs, regardless of the foreign eggs' colours. We tested hosts' responses to gradients of natural (blue-green to brown) and artificial (green to purple) egg colours, and demonstrate that hosts base rejection decisions on both the direction and degree of colour dissimilarity along the natural, but not artificial, gradient of egg colours. Hosts rejected brown eggs and accepted blue-green eggs along the natural egg colour gradient, irrespective of the total perceived dissimilarity from their own egg's colour. By contrast, their responses did not vary along the artificial colour gradient. Our results demonstrate that egg recognition is specifically tuned to the natural gradient of avian eggshell colour and suggest a novel decision rule. These results highlight the importance of considering sensory reception and decision rules when studying perception, and illustrate that our understanding of recognition processes benefits from examining natural variation in phenotypes.

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

“…Hanley et al. () 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

“…This implies that hosts set thresholds along phenotypic extrema on each side of a phenotypic range ('multiple thresholds'), in this case, eggs that are very blue -green and brown. However, a recent study [17] on two species of thrushes (Turdus spp.) found that this was not the case.…”