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.
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Pairing status moderates both the production of and responses to anti‐parasitic referential alarm calls in male yellow warblers
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 (
- NSF-PAR ID:
- 10452191
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ethology
- Volume:
- 127
- Issue:
- 5
- ISSN:
- 0179-1613
- Page Range / eLocation ID:
- p. 385-394
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Referential alarm calls that denote specific types of dangers are common across diverse vertebrate lineages. Different alarm calls can indicate a variety of threats, which often require specific actions to evade. Thus, to benefit from the call, listeners of referential alarm calls must be able to decode the signaled threat and respond to it in an appropriate manner. Yellow warblers ( Setophaga petechia ) produce referential “seet” calls that signal to conspecifics the presence of nearby obligate brood parasitic brown-headed cowbirds ( Molothrus ater ), which lay their eggs in the nests of other species, including yellow warblers. Our previous playback experiments have found that red-winged blackbirds ( Agelaius phoeniceus ), a species also parasitized by brown-headed cowbirds, eavesdrop upon and respond strongly to yellow warbler seet calls during the incubation stage of breeding with aggression similar to responses to both cowbird chatters and predator calls. To assess whether red-winged blackbird responses to seet calls vary with their own risk of brood parasitism, we presented the same playbacks during the nestling stage of breeding (when the risk of brood parasitism is lower than during incubation). As predicted, we found that blackbirds mediated their aggression toward both cowbird chatter calls and the warblers’ anti-parasitic referential alarm calls in parallel with the low current risk of brood parasitism during the nestling stage. These results further support that red-winged blackbirds flexibly respond to yellow warbler antiparasitic referential calls as a frontline defense against brood parasitism at their own nests.more » « less
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Yellow warblers ( Setophaga petechia ) use referential ‘seet’ calls to warn mates of brood parasitic brown-headed cowbirds ( Molothrus ater ). In response to seet calls during the day, female warblers swiftly move to sit tightly on their nests, which may prevent parasitism by physically blocking female cowbirds from inspecting and laying in the nest. However, cowbirds lay their eggs just prior to sunrise, not during daytime. We experimentally tested whether female warblers, warned by seet calls on one day, extend their anti-parasitic responses into the future by engaging in vigilance at sunrise on the next day, when parasitism may occur. As predicted, daytime seet call playbacks caused female warblers to leave their nests less often on the following morning, relative to playbacks of both their generic anti-predator calls and silent controls. Thus, referential calls do not only convey the identity or the type of threat at present but also elicit vigilance in the future to provide protection from threats during periods of heightened vulnerability.more » « less
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Abstract Nest‐sharer avian brood parasites do not evict or otherwise kill host chicks, but instead inflict a range of negative effects on their nestmates that are mediated by interactions between the parasite and host life history traits. Although many of the negative fitness effects of avian brood parasitism are well documented across diverse host species, there remains a paucity of studies that have examined the impacts of parasitism across the entirety of host ontogeny (i.e., from when an egg is laid until independence). More specifically, few studies have examined the impact of brood parasitism on the pre‐ and post‐fledging development, physiology, behavior, and survival of host offspring. To help fill this knowledge gap, we assessed the effects of brood parasitism by Brown‐headed Cowbirds (
Molothrus ater ) across the ontogeny (incubation, nestling, and post‐fledging period) of nine sympatrically breeding host species in central Illinois, USA; due to sample sizes, impacts on the post‐fledging period were only examined in two of the nine species. Specifically, we examined the impact of brood parasitism on ontogenetic markers including the embryonic heart rate, hatching rate, nestling period length, nest survival, and offspring growth and development. Additionally, in species in which we found negative impacts of cowbird parasitism on host nestmate ontogeny, we examined whether the difference in adult size between parasites and their hosts and their hatching asynchrony positively predicted variation in host costs across these focal taxa. We found that costs of cowbird parasitism were most severe during early nesting stages (reduction in the host clutch or brood size) and were predicted negatively by host size and positively by incubation length. In contrast, we only found limited costs of cowbird parasitism on other stages of host ontogeny; critically, post‐fledging survival did not differ between host offspring that fledged alongside cowbirds and those that did not. Our findings (i) highlight the direct costs of cowbird parasitism on host fitness, (ii) provide evidence for when (the stage) those costs are manifested, and (iii) may help to explain why many anti‐cowbird defenses of hosts have evolved for protection from parasitism during the laying and incubation stages. -
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Abstract To avoid mobbing attacks by their hosts during egg laying, some avian brood parasites have evolved traits to visually and/or acoustically resemble predator(s) of their hosts. Prior work established that reed warblers (
Acrocephalus scirpaceus ), a small host species of the brood parasitic common cuckoo (Cuculus canorus ), delayed returning to the nest when confronted by either the calls of the female cuckoo or that of the predatory sparrowhawk (Accipiter nisus ). It remains less clear, however, whether female cuckoo calls also suppress the nest defences of larger and more aggressive hosts. Such hosts typically attack vigorously, and can even hurt the brood parasitic intruders, instead of fleeing in the face of danger. Here, we tested whether the female cuckoo calls dampen mobbing intensity in a much largerAcrocephalus host of the common cuckoo, the great reed warbler (A. arundinaceus ). We presented great reed warbler pairs with female common cuckoo models at their nests without and then with playing back the female‐specific bubbling calls of the cuckoo. As controls, we tested the hosts’ responses to harmless collared dove (Streptopelia decaocto ) models, also without and then with the playbacks of dove calls. We found that the playback of female brood parasite calls reduced the aggression of hosts towards the cuckoo models as compared to model presentations without female calls, but we detected no such effect of the control calls with dove models. Our results revealed that female cuckoo calls effectively suppress the antiparasitic responses of great reed warbler hosts, which could aid parasites to approach the nest undiscovered and to evade the costly attacks of this large host. Therefore, the female call can be regarded as a general part of the cuckoo's trickery repertoire for successful parasitism.
