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

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.

The American robin (Turdus migratorius): A focal species for anti‐parasitic egg rejection studies among hosts of the brown‐headed cowbird (Molothrus ater)

“…Experiments conducted with cylindrical-or cuboid-shaped white model eggs increased rejection rates by 59.5% (Underwood & Sealy, 2006). Similarly, the likelihood of egg rejection increased when panel length and width of model eggs were stretched creating a series of increasingly non-egg like shapes of robin-blue painted models (Hauber, Winnicki, et al, 2021). Deviations from natural egg shapes could be perceived as nest debris and elicit prompt sanitation behaviors similar to the removal of leaves, flowers, broken eggshells, or fecal sacs from the nest cup (Guigueno and Sealy 2012).…”

“…Small, subtle variations in egg shape could be perceived by visual and/or tactile modalities, but are not predictors of egg rejection when the model egg closely resembles the overall size of natural cowbird eggs (Igic et al, 2015;Underwood & Sealy, 2006). In turn, clearly non-egg like egg shapes are associated with high rejection rates (Hauber, Winnicki, et al, 2021;Underwood & Sealy, 2006).…”

“…Many of the studies in our review utilized artificial model eggs when conducting rejection experiments. Previous studies on egg rejection used model eggs constructed from plastic, wood, plaster of paris, and, most recently, 3D-printed natural plastic eggs that offer a more precise design that can be easily manipulated (Hauber, Winnicki, et al, 2021;Igic et al, 2015). Despite these advantages of 3D printing, there was no difference in rejection rates based on the type of model egg used (plaster versus.…”

A Meta-Analysis of Avian Egg Traits Cueing Egg-Rejection Defenses Against Brood Parasitism

“…Finally, we predicted that hosts of Old World cuckoos better discriminate by color and maculation relative to other traits than cowbird hosts due to several million years longer coevolutionary experience with more mimetic parasite eggs in the former group of hosts (Caves et al, 2017;Krüger and Pauli, 2017). We make this prediction because we know from prior research that egg rejection belongs to a different class of recognition systems compared to other recognition tasks faced by nesting birds (e.g., nest hygiene: Hauber et al, 2021).…”

Host and brood parasite coevolutionary interactions covary with comparative patterns of the avian visual system

“…Accordingly, we assessed the following predictions of our novel Coevolved Sensory System Hypothesis as applied to eye size across a large sample of avian lineages: (i) eyes of brood parasites should be larger than those of hosts as many parasites use visual cues of host breeding activity to locate nests for future parasitism [ 18 , 19 ], (ii) hosts should have smaller eyes than non-hosts if parasites use potential host species with lower than expected visual sensitivities and reduced ability to discriminate among objects in the nest, including the recognition of foreign eggs [ 20 ], and eye size of hosts should positively covary with both (iii) parasite-egg rejection patterns by the same species and (iv) the extent of avian-perceivable, visible host-eggshell mimicry by parasites.…”