Eavesdropping on Referential Yellow Warbler Alarm Calls by Red-Winged Blackbirds Is Mediated by Brood Parasitism Risk

Abstract

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.

Red‐winged blackbirds nesting nearer to yellow warbler and conspecific nests experience less brood parasitism

“…Other host co‐occurring species may need to be considered to test the mutualistic nest neighbor hypothesis, which we did neither here nor in previous studies (Lawson et al, 2020 ; Lawson, Enos, Gill, & Hauber, 2021 ). If no other host species responds aggressively to warbler seet calls or benefit from nesting near warblers, then this would provide some more evidence of a mutualistic relationship between red‐winged blackbirds and yellow warblers specifically.…”

“…The mechanism by which interspecific benefits act was not directly tested, but the lower parasitism risk may be due to better detection of the warblers' referential alarm calls that could inform blackbirds about cowbird presence on their own territories. Indeed, our previous research showed that blackbirds eavesdrop on and respond to warbler seet calls with their own nest defense behaviors to prevent brood parasitism (Lawson et al, 2020 ) and that expression of defensive behaviors varied with current brood parasitism risk on the blackbirds' nests within their own territories (Lawson, Enos, Gill, & Hauber, 2021 ). Taken together with our previous research, we suggest that blackbirds use warbler seet calls produced by nearby neighbors as an early warning system to improve vigilance as well as defense at their own nests.…”

“…The referential “seet” call is produced by these warblers in response to cowbird cues (such as cowbird vocalizations or model presentations), and females that hear this call react as they would to a cowbird, by returning to and positioning themselves upon their nests to prevent the cowbird from inspecting and/or parasitizing the nest (Gill & Sealy, 2004 ). Recently we discovered that the red‐winged blackbird ( Agelaius phoeniceus , hereafter “blackbird”), another cowbird host species that often nests near warbler nests, eavesdrops on seet calls and responds with similar aggression toward both playbacks of warbler's seet calls and playbacks of cowbird chatter calls (Lawson et al, 2020 ; Lawson, Enos, Gill, & Hauber, 2021 ).…”

Ontogenetic effects of brood parasitism by the Brown‐headed Cowbird on host offspring

“…Such findings are also consistent with costs found in other nest‐sharer brood parasite systems, such as in Shiny Cowbird ( Molothrus bonariensis ; Massoni & Reboreda, 1998), Bronzed Cowbird ( Molothrus aeneus ; Carter, 1986), and Great Spotted Cuckoo ( Clamator glandarius ; Soler et al, 1997), in which host eggs are commonly cracked or removed during parasitism. Thus, our findings help explain why many hosts' anti‐parasitic defenses are front‐loaded and geared toward earlier nesting stages (e.g., preventing parasitic egg laying; Lawson et al, 2021; Peer et al, 2005), as defenses evolved during subsequent stages of ontogeny may only mitigate further costs of parasitism rather than preventing the primary, direct losses of fitness from a reduction in brood size.…”

“…Lawson et al, 2021; T A B L E 2 Results from multistate models examining the effect of parasitism by Brown-headed Cowbirds (BHCO) on age-specific rates of post-fledging survival in Common Yellowthroats (n = 37) and Dickcissels (n = 102) of central Illinois,USA, 2014USA, -2019 …”