Pairing status moderates both the production of and responses to anti‐parasitic referential alarm calls in male yellow warblers

Abstract

Defending offspring incurs temporal and energetic costs and can be dangerous for the parents. Accordingly, the intensity of this costly behavior should reflect the perceived risk to the reproductive output. When facing costly brood parasitism by brown‐headed cowbirds (Molothrus ater), where cowbirds lay eggs in heterospecific nests and cause the hosts to care for their young, yellow warblers (Setophaga petechia) use referential “seet” calls to warn their mates of the parasitic danger. Yellow warblers of both sexes produce this call only in response to cowbirds or seet‐calling conspecifics. Seet calls are mainly produced during the laying and incubation stages of breeding, when risk of brood parasitism is highest, rather than during the nestling stage. On the other hand, general alarm calls (chips) are produced throughout the nesting cycle and are also used in conspecific interactions unrelated to nesting. We hypothesized that context shapes responses prior to breeding as well, such that yellow warblers without a mate and active nest would be less likely to respond to playbacks that simulate brood parasitism risk. To test this hypothesis, we presented playbacks of two nest threats, cowbirds (brood parasite) and blue jays (Cyanocitta cristata; nest predator), on territories of unmated male warblers (unpaired) and male warblers with a known mate (paired). We found that unpaired males were unresponsive toward playbacks indicating nest threats, whereas paired males were significantly more aggressive and vocal toward these playbacks compared to control playbacks. However, both paired and unpaired males were vocally responsive toward chip calls, which are informative for males regardless of pairing status. Male yellow warblers appear to adjust their responses during the earliest stages of breeding depending on the contextual relevance of specific threat stimuli, and together with prior studies, our work further supports that referential seet calls are associated with stage‐specific risk of brood parasitism.

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

“…However, cowbird presence during cowbird playbacks did not statistically covary with whether warblers seet called in response to the chatter playback (Fisher's exact test, p = .37), indicating that cowbird chatters, like cowbird models, are a suitable stimulus to experimentally simulate cowbirds. Female warblers seet called equally during cowbird chatter playbacks than during seet playbacks, but in our playback study with male yellow warblers (Lawson et al, 2021), we found that males seet called at a higher rate towards cowbird chatters compared to seet call playbacks. Personal (private) information (sensu Thorogood & Davies, 2012) about brood parasitism risk, such as directly seeing or hearing the brood parasite, can offer greater reliability of the threat compared to social information obtained from assessing cowbird presence based on seet calls from neighbors.…”

“…Numerous published replication and pivoting experimental studies in the yellow warbler/brownheaded cowbird system have led to an integrated understanding of the seet call as a referential alarm call used in antiparasitic nest defense and the socio-environmental contexts that promote its use (see Appendix). Together with our recent works (Lawson et al, 2020(Lawson et al, , 2021, our study here is amongst the first to use solely acoustic stimuli to compare female yellow warblers' aggressive responses towards a full series of brood parasite, nest predator, and conspecific alarm vocalizations. Indeed, previous studies in this system used mostly either combinations of acoustic/visual stimuli or only visual stimuli.…”

“…Here we conducted our own quasi-replication study in the yellow warbler/brown-headed cowbird system using acoustic presentations as our sole experimental stimuli. We present a novel, playback-only based experimental design to probe anti-parasitic responses of female (this study) and male yellow warblers (Lawson et al, 2021), as well as heterospecific red-winged blackbirds (Agelaius phoeniceus, Lawson et al, 2020). Accordingly, during laying/incubating stages at yellow warbler nests, we presented playbacks of female cowbird chatters, conspecific seet calls, conspecific chip calls, nest predator calls (blue jay, Cyanocitta cristata), as well as procedural controls of songs of a harmless sympatric heterospecific, the wood thrush (Hylocichla mustelina).…”

Quintuple parasitism of a great reed warbler nest by common cuckoos

“…Once hatched, the brood parasitic chicks greatly reduce the breeding success of the hosts, by outcompeting the hosts' offspring or by killing their nestmates altogether (Davies, 2000). Therefore, hosts have evolved multifaceted defenses to avoid being parasitized (Feeney et al, 2012;Welbergen & Davies, 2009), such as building highly concealed nests (Jelínek et al, 2014;Mérő & Žuljević, , 2019, gathering social information regarding the presence of brood parasites (Lawson et al, 2020(Lawson et al, , 2021Thorogood & Davies, 2012, and by mobbing adult brood parasites (Požgayová et al, 2009;Trnka et al, 2013). Brood parasites bypass these defenses by fast egg-laying (Jelínek et al, 2021), color dimorphism (Thorogood & Davies, 2012), visual and acoustic mimicry (Marton et al, 2021;Trnka & Prokop, 2012;Welbergen & Davies, 2008;York & Davies, 2017), and sometimes also by retaliatory strikes and brood destruction (Soler et al, 1995).…”

Referential alarm calling elicits future vigilance in a host of an avian brood parasite

“…Yellow warblers ( Setophaga petechia , hereafter ‘warblers’) express a suite of anti-parasitic behaviours, including referential signals [5,7]. These ‘seet’ calls refer specifically to brood parasitic brown-headed cowbirds ( Molothrus ater , hereafter ‘cowbirds’), whereas ‘chip’ calls generically alert mates to diverse predators and intruding conspecifics [5,8]. In response to experimental exposure to either parasitic stimuli (e.g.…”