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

Peter Samaš,Marcel Honza,Mark E Hauber

doi:10.3389/fevo.2021.703208

Abstract

The capability of hosts to reject the odd egg from their nest is one of the key defenses against avian brood parasitism. Considerable research effort has been devoted to exploring which phenotypic traits of eggshells facilitate to cue the recognition of the parasitic egg. Here we have reviewed studies addressing salient egg traits involved in the rejection of foreign eggs and used a formal meta-analysis to quantify their relative importance. Hosts appear to rely to a large extent on eggshell color traits, followed by maculation patterns. Hosts respond with similar rates of egg rejection to natural vs. model eggs and when breeding in both closed and open nests. Analyses of experiments on hosts of Cuculus and Molothrus parasites, the two best studied brood parasitic lineages with different co-evolutionary histories, yield similar conclusions. We also identify several poorly studied potential egg recognition cues, such as odor or weight, and recommend exploring even the visual traits in more detail, including chromatic and achromatic contrasts or experimentally manipulated egg maculation characteristics. Recent technological and sensory ecological advances open many new research avenues to experimentally examine the role of diverse egg characteristics in antiparasite defenses.

Highlights

The capability to perceive, recognize, and reject the parasitic egg(s) is a critical defense in hosts of avian brood parasites, which lay their eggs into the nests of other birds (Davies, 2000)
We found significant overall effect on egg rejection for egg color (Cohen’s h = 1.24, 89% credible intervals = 0.98–1.49), followed by maculation (h = 0.69 [0.45–0.94]) and UV (h = 0.43 [0.16– 0.69]) (Figure 1 and Supplementary Table 1)
Pairwise comparisons within each egg trait type according to stimulus type were similar except of egg size trait

Summary

Introduction

The capability to perceive, recognize, and reject the parasitic egg(s) is a critical defense in hosts of avian brood parasites, which lay their eggs into the nests of other birds (Davies, 2000). Some lineages of brood parasites have evolved sophisticated eggshell mimicry to fool the hosts, which in turn, have evolved fine-tuned abilities to discriminate and reject the foreign egg. To combat host defenses, some lineages of brood parasites have evolved sophisticated eggshell mimicry to fool the hosts, which in turn, have evolved fine-tuned abilities to discriminate and reject the foreign egg This suite of antiparasite defense behaviors has attracted considerable observational, comparative, and experimental research attention in the last decades (e.g., Grim, 2007; Medina and Langmore, 2015), albeit the first such experiments had been performed by naturalists already more than a century ago (reviewed in Sealy and Underwood, 2012). Experiments usually involve adding to or exchanging one or more foreign eggs in the host nest and observing the host’s reaction. Stephen Rothstein was a pioneer of egg rejection experimentation (e.g., Rothstein, 1970), and his methods are still used by many researchers with only minor modifications (e.g., Canniff et al, 2018; Luro et al, 2018).

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Conclusion