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

Abstract

Obligate avian brood parasites lay their eggs in the nest of other species and impose reproductive costs upon their hosts. In response, many hosts have evolved a variety of defenses to avoid or reduce the costs associated with parasitism, including the rejection of parasitic eggs. Here, we review the different sensory and contextual cues involved in egg rejection by the American robin (Turdus migratorius), one of the handful of rejecter host species of the brood parasitic brown-headed cowbird (Molothrus ater) in North America. Robins are an open-cup nesting species whose eggs dramatically differ in appearance from the cowbirds' eggs; therefore, we expect that visual cues would be the primary sensory modality used when rejecting foreign eggs. The results of our comprehensive review confirm that visual cues, including eggshell color and maculation, are strong predictors of robins' egg rejection responses. Egg size, egg shape, and other contextual traits (including clutch composition, maternal status, and sympatry with the parasite) also affect egg rejection in this host species. Future research into this species' egg rejection mechanisms would benefit from comparisons to other strongly and moderately egg rejecter host species of brown-headed cowbirds, as well as from seeking to understand further why so many hosts of this particular parasitic lineage accept foreign eggs. We also discuss the implications of this review for other egg rejecter and non-rejecter hosts of diverse avian brood parasites.

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

“…of parasitic egg in the host nest (Honza and Cherry, 2017;Turner and Hauber, 2021). All the three visually-related eggshell traits, including color, maculation, and UV, showed no overlap with null effect sizes.…”

“…Taken together, the experiments also imply that specific eggshell traits differ in their importance for the recognition and rejection processes (Honza and Cherry, 2017). However, there are still missing quantitative estimates on overall eggshell characteristics and their effects on host behaviors that prevent us from further exploring and discussing their significance for egg rejection as an antiparasitic strategy (Turner and Hauber, 2021). Knowledge of the latter will help us to better understand the cognitive processes involved in the brood parasite -host coevolution and also to design informative future experiments to fill in the missing gaps.…”

“…Other hypothesized recognition cues, such as egg odor (e.g., Soler et al, 2014;Hauber, 2020) or weight (Ruiz-Raya et al, 2015) could not be statistically analyzed due to insufficient number of published reports (Figure 5). Therefore, our first recommendation is that more such studies address the potential roles of tactile-only or olfactory cued egg rejection behaviors in varied hosts of diverse avian brood parasites (also see Turner and Hauber, 2021). Finally, even in the studies performing a valid egg experiment, using a control treatment was particularly limiting selection criterion and led to a notable severe reduction of the selected studies for final analyses (Supplementary Table 1).…”

“…On the other hand, model (artificial) stimuli can be especially advantageous when planning a carefully designed experiment allowing exact alterations of the focal traits (Igic et al, 2015;Yang et al, 2019). Several review studies already examined the effect of egg type stimuli on their rejection probabilities (Honza and Cherry, 2017;Turner and Hauber, 2021) but we are still missing a comprehensive survey estimating such stimuli's effects in a standardized comparison.…”

“…In this study, we have built upon the previous review by Honza and Cherry (2017) with the aim to provide a formal meta-analysis through a quantitative measure of the magnitude of the experimental effect (effect sizes) for the egg characteristics involved in the recognition of parasitic egg in the host nest. Such a quantitative assessment across multiple host species and lineages of diverse parasitic species and lineages has not yet been conducted, although qualitative reviews of experiments on individual species' egg rejection cues have begun to appear in the published literature (e.g., Turner and Hauber, 2021).…”