Natural and artificial scents do not increase egg rejection rates of model brood parasitic eggs by American robins (Turdus migratorius)

Abstract

Hosts of obligate avian brood parasites can diminish or eliminate the costs of parasitism by rejecting foreign eggs from the nests. A vast literature demonstrates that visual and/or tactile cues can be used to recognize and reject natural or model eggs from the nests of diverse host species. However, data on olfaction-based potential egg recognition cues are both sparse and equivocal: experimentally-applied, naturally-relevant (heterospecific, including parasitic) scents do not appear to increase egg rejection rates in two host species, whereas unnatural scents (human and tobacco scents) do so in one host species. Here I assessed the predictions that (i) human handling of mimetically-painted model eggs would increase rejection rates, and (ii) applying unnatural or natural scents to mimetically or non-mimetically painted model eggs alters these eggs’ respective rejection rates relative to controls. I studied wild American Robins (Turdus migratorius), a robust rejecter species of the eggs of obligate brood parasitic Brown-headed Cowbirds (Molothrus ater). There was no statistical evidence to support either prediction, whereas poorer color-mimicry was still a predicted cause of greater egg rejection in this data set. Nonetheless, future studies could focus on this and other host species and using these and different methods to apply and maintain the scenting of model eggs to more directly test hosts’ use of potential olfactory cues in the foreign-egg rejection process.

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

“…Few studies have examined how the scent of the parasitic eggs can inform rejection decisions by rejecter hosts (but see Rasmussen 2013; Soler et al., 2014). The only study that has tested the effect of scent in robins found no effect of scent treatments (artificial: citrus, natural: cowbird cloacal lavage, control: solvent only) on egg rejection responses (Hauber, 2020).…”

“…Little is known about the general role of olfactory cues in anti‐parasitic egg recognition (but see Rasmussen 2013; Soler et al., 2014). From our review of robins' responses to (model) cowbird eggs, olfaction did not influence egg rejection in robins; in the only study analyzing olfactory cues that we sourced for this review, neither unnatural scents of model eggs (citrus, human) nor a natural scent (cowbird cloacal lavage) affected egg rejection in robins (Hauber, 2020). This was contrary to findings in magpies, where Soler et al., (2014) found that the unnatural scent treatment of human handling or tobacco smoke both significantly increased egg rejection rates, although natural scent treatments of uropygial secretion of cuckoos and cloacal scents of cuckoos and magpies did not do so.…”

“…Other cowbird‐egg rejecter host species may have different egg characteristics, rejection methods, and life histories that necessitate the use of different cues when rejecting foreign eggs (cowbird and model eggs (Peer & Sealy, 2004)). For instance, olfactory cues did not significantly affect egg rejection in robins (Hauber, 2020), whereas foreign scents or unusual tactile qualities of nest contents could be more important cue for rejecter species whose eggs are closely mimicked by the parasite, or species with domed or cavity nests whose lighting milieu makes visual inspection of the eggs more difficult compared to open‐cup nesting hosts. Therefore, the extensive literature we have gathered on egg, maternal, and habitat traits influencing egg rejection in robins should be replicated in other rejecter species of cowbirds and, in general, of other brood parasitic species (Hauber, Hoover, et al., 2021).…”

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

“…Only partly vision-related traits such as egg shape and size showed small effects and the effect of other, also potentially partly tactile traits represented by natural vs. model materials, was negligible. 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).…”

Ground nesting by arboreal American robins (Turdus migratorius)

“…As part of our ongoing studies of robins and their anti-parasitic egg rejection behaviors (e.g., Abolins-Abols & Hauber, 2020;Hauber, 2020), from 2018 to 2020, we located 1,170 robin nests at a commercial deciduous tree farm from April to July in Champaign County, Illinois, USA, including 24 robin nests built directly on the ground.…”