Influence of Shape on Egg Discrimination in American Robins and Gray Catbirds

Abstract

The eggs of some obligate brood parasites are more spherical than the eggs of their non‐parasitic relatives and hosts, which contributes to the increased strength of their shells. We examined whether egg shape, including the more spherical shape of brown‐headed cowbird eggs (Molothrus ater), influenced egg discrimination in American robins (Turdus migratorius) and gray catbirds (Dumetella carolinensis). We added a series of artificial objects to robin and catbird nests that varied in shape from a control host egg, a rounded, cowbird‐like egg, to odd‐shaped objects. Real cowbird eggs were significantly more spherical than catbird and robin eggs, which confirmed a potential cue for egg recognition. Object shape significantly influenced the probability of rejection and time to rejection in both robins and catbirds. However, rounded eggs and spheres were rejected infrequently and at frequencies similar to control eggs. Therefore, the shape of a brood parasite's egg does not appear to influence egg discrimination in these two rejecters. Robins and catbirds rejected significantly more odd‐shaped objects than egg‐shaped objects and odd‐shaped objects were rejected significantly sooner than egg‐shaped objects. The rejection of odd‐shaped objects likely represents an expression of nest‐sanitation behaviour where debris is removed from the nest. By comparison with other studies of accepters of cowbird eggs, robins and catbirds appear to reject higher proportions of odd‐shaped objects, which suggests they may have more refined abilities to discriminate against foreign objects in their nests.

Does nest sanitation elicit egg rejection in an open-cup nesting cuckoo host rejecter?

“…In addition, some hosts do not practice nest sanitation, but can reject some eggs (Peer 2017). Underwood and Sealy (2006) suggested that the ability of American Robins and Gray Catbirds (Dumetella carolinensis) to recognize non-egg objects was significantly higher than that of other species without egg discrimination ability. Moskát et al (2003) examined the relationship between nest sanitation and egg rejection in the Great Reed Warbler (Acrocephalus arundinaceus) and showed that the rejection rates of the non-egg models were significantly higher than those of cuckoo model eggs, either during hatching or in the early stage of incubation.…”

“…However, egg rejection by hosts may be an extension of their nest sanitation behavior (Rothstein 1975;Guigueno and Sealy 2009;Yang et al 2015b). Many studies have shown the relationship between the nest sanitation behavior and egg discrimination ability of cuckoo hosts using various nonovoid foreign objects (Ortega and Cruz 1988;Moskát et al 2003;Underwood and Sealy 2006;Yang et al 2015a, b;Luro and Hauber 2017;Peer 2017;review Moskát et al (2003) proposed that the less similar the shape of a foreign object to a cuckoo egg, the higher the host rejection rate, so that the ability of the host to recognize a cuckoo egg is an extension of its nest sanitation behavior. In addition, hosts with strong egg discrimination ability tend to be more capable of rejecting non-ovoid objects than are hosts without egg discrimination ability (Yang et al 2015b), indicating that host egg discrimination ability is associated with nest sanitation behaviour (Underwood and Sealy 2006;Guigueno and Sealy 2009;Yang et al 2015a).…”

“…Nest sanitation behavior is the removal of foreign objects from the nest, and is a very common behavior in breeding birds (Alvarez et al 1976;Moskát et al 2003;Underwood and Sealy 2006;Guigueno andSealy 2009, 2017). This behavior can improve the incubation efficiency of the parent birds, reduce physical damage to eggs from foreign objects, lower the chance of egg infection by pathogenic bacteria, and decrease the risk of predation (Guigueno and Sealy 2012).…”

Nest sanitation in passerine birds: implications for egg rejection in hosts of brood parasites

“…Studies involving experimental addition of objects have given insight into the genetic basis of nest sanitation and its relationship with the rejection responses of some hosts of brood parasites (Smith 1943;Bártol et al 2003;Moskát et al 2003;Underwood and Sealy 2006a). The subject has not been reviewed, despite that the study of nest sanitation has drawn increased interest (Ortega and Cruz 1988;Ortega et al 1993;Bártol et al 2003;Moskát et al 2003;Underwood and Sealy 2006a;Guigueno and Sealy 2009).…”

“…The removal of non-egg-shaped objects has been considered a type of nest-sanitation behavior (Alvarez et al 1976;Bártol et al 2003;Moskát et al 2003;Underwood and Sealy 2006a;Guigueno and Sealy 2009). Studies involving experimental addition of objects have given insight into the genetic basis of nest sanitation and its relationship with the rejection responses of some hosts of brood parasites (Smith 1943;Bártol et al 2003;Moskát et al 2003;Underwood and Sealy 2006a).…”