Responses of Female Yellow Warblers to Playbacks Signaling Brood Parasitism or Predation Risk: A Quasi-Replication Study

Shelby L Lawson,Sharon A Gill,Niko C Mendes,Janice K Enos,Mark E Hauber

doi:10.26451/abc.08.02.08.2021

Abstract

Many avian species are negatively impacted by obligate avian brood parasites, which lay their eggs in the nests of host species. The yellow warbler (Setophaga petechia), which is host to the brood-parasitic brown-headed cowbird (Molothrus ater), represents one of the best-replicated study systems assessing antiparasitic host defenses. Over 15 prior studies on yellow warblers have used model-presentation experiments, whereby breeding hosts are exposed to models of brown-headed cowbirds or other nest threats, to test for anti-parasitic defenses unique to this species. Here we present results from our own quasi-replication study of the yellow warbler/brown-headed cowbird system, which used a novel design compared to previous experiments by pivoting to conduct acoustic playback treatments only, rather than presenting visual models with or without calls. We exposed active yellow warbler nests to playbacks of brown-headed cowbird chatters (brood parasite), blue jay (Cyanocitta cristata; nest predator) calls, conspecific “seet” calls (a referential alarm call for brood parasitism risk), conspecific “chip” calls (a generic alarm call), or control wood thrush (Hylocichla mustelina; harmless heterospecific) songs during the incubation stage. Similar to previous studies, we found that female yellow warblers seet called more frequently in response to playbacks of both brood parasitic chatter calls and conspecific seet calls whereas they produced more chip calls in response to the playback of nest predator calls. In contrast, female yellow warblers approached all playbacks to similar distances, which was different from the proximity patterns seen in previous studies. Our study demonstrates the importance of both replicating, and also pivoting, experimental studies on nest defense behaviors, as differences in experimental design can elicit novel behavioral response patterns in the same species.

Highlights

The fitness of over 200 North American passerine species is known to be negatively impacted by the brown-headed cowbird (Molothrus ater, hereafter “cowbird”), an obligate avian brood parasite that lays its eggs in the nests of other species, leaving the host to care for the parasitic young (Davies, 2010)
The yellow warbler (Setophaga petechia), a common host for cowbirds, represents one of the best replicated study systems assessing behavioral defenses related to host-parasite interactions
Response and its Latency Based on the binomial model, female yellow warblers differed in whether they responded to playbacks depending on treatment (treatment term: F(4, 83) = 7.27, p < .001; Figure 1)

Summary

Introduction

The fitness of over 200 North American passerine species is known to be negatively impacted by the brown-headed cowbird (Molothrus ater, hereafter “cowbird”), an obligate avian brood parasite that lays its eggs in the nests of other species, leaving the host to care for the parasitic young (Davies, 2010). Several early studies on yellow warbler antiparasitic nest defenses noted aggressive responses from both sexes towards models (in particular, of adult females) of brown-headed cowbirds presented to host pairs at the nest, given that female cowbirds represent the most immediate brood parasitic threat for foreign-egg laying (Folkers & Lowther, 1985; Robertson & Norman, 1976a, b) This experimental approach was expanded by studies that tested yellow warblers’ responses to female cowbird models during different stages of nesting and found that aggression was stronger during laying and incubation (relative to the nestling stage), when hosts are at the highest risks of being successfully parasitized (Burgham & Picman, 1989; Hobson & Sealy, 1989a). We predicted that female warblers would 1) produce seet calls exclusively in response to cowbird chatter and seet call playbacks, 2) produce more chip calls in response to blue jay calls compared to other playbacks, and 3) respond more quickly and approach all playbacks more closely than the control wood thrush playback

Methods

Results

Conclusion