The limits of egg recognition: testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest.

Ian R Hays,Mark E Hauber,Sarah K Winnicki,Daniel Hanley,Jeffrey P Hoover

doi:10.1098/rsos.201615

Abstract

Some hosts of avian brood parasites reduce or eliminate the costs of parasitism by removing foreign eggs from the nest (rejecter hosts). In turn, even acceptor hosts typically remove most non-egg-shaped objects from the nest, including broken shells, fallen leaves and other detritus. In search for the evolutionary origins and sensory mechanisms of egg rejection, we assessed where the potential threshold between egg recognition and nest hygiene may lie when it comes to stimulus shape. Most previous studies applied comparisons of egg-sized objects with non-continuous variation in shape. Here, instead, we used two series of three-dimensional-printed objects, designed a priori to increasingly diverge from natural eggs along two axes (width or angularity) of shape variation. As predicted, we detected transitions from mostly acceptance to mostly rejection in the nests of American robins Turdus migratorius along each of the two axes. Our methods parallel previous innovations in egg-rejection studies through the use of continuous variation in egg coloration and maculation contrast, to better understand the sensory limits and thresholds of variation in egg recognition and rejection in diverse hosts of avian brood parasites.

Highlights

Most birds keep their nests relatively clean, at least during the incubation stage, by removing detritus and broken eggs/shells from the nest cup [1]; such nest hygiene behaviour has been the focus of extensive research, including some of the classic work by Nobel-prize winning Niko Tinbergen [2,3]
Hosts of avian brood parasites exhibit a wide range of egg-rejection abilities [6,7], which provides a tractable system for studying how birds recognize eggs from non-egg objects
Egg recognition can be complex as there are both costly trade-offs and physical-cognitive constraints accompanying this behaviour

Summary

Introduction

Most birds keep their nests relatively clean, at least during the incubation stage, by removing detritus (e.g. fallen leaves or flower petals) and broken eggs/shells from the nest cup [1]; such nest hygiene behaviour has been the focus of extensive research, including some of the classic work by Nobel-prize winning Niko Tinbergen [2,3]. By removing the parasitic egg(s), hosts forgo reproductive costs accrued when raising unrelated chick(s) in their brood [8,9]. Egg rejection can be costly because hosts may mistakenly reject (rejection error: [10,11]) or damage their own eggs (rejection cost: [12,13]) when attempting to pierce and remove thicker-shelled parasitic eggs [14,15]. Hosts may face mechanical constraints if they are physically unable to pierce or grasp and lift foreign eggs too thick or heavy from the nest [18,19]. How hosts of brood parasites balance the benefits and costs of removing foreign versus own eggs from the nest remains an area of active research [20]

Methods

Results

Conclusion