Responses of naive yellow warblers to a novel nest predator

Avian vocalisations: the female perspective

Research on avian vocalisations has traditionally focused on male song produced by oscine passerines. However, accumulating evidence indicates that complex vocalisations can readily evolve outside the traditional contexts of mate attraction and territory defence by male birds, and yet the previous bias towards male song has shaped - and continues to shape - our understanding of avian communication as a whole. Accordingly, in this review we seek to address this imbalance by synthesising studies on female vocalisations from across signalling contexts throughout the Aves, and discuss the implications of recent empirical advances for our understanding of vocalisations in both sexes. This review reveals great structural and functional diversity among female vocalisations and highlights the important roles that vocalisations can play in mediating female-specific behaviours. However, fundamental gaps remain. While there are now several case studies that identify the function of female vocalisations, few quantify the associated fitness benefits. Additionally, very little is known about the role of vocal learning in the development of female vocalisations. Thus, there remains a pressing need to examine the function and development of all forms of vocalisations in female birds. In the light of what we now know about the functions and mechanisms of female vocalisations, we suggest that conventional male-biased definitions of songs and calls are inadequate for furthering our understanding of avian vocal communication more generally. Therefore, we propose two simple alternatives, both emancipated from the sex of the singer. The first distinguishes song from calls functionally as a sexually selected vocal signal, whilst the second distinguishes them mechanistically in terms of their underlying neurological processes. It is clear that more investigations are needed into the ultimate and proximate causes of female vocalisations; however, these are essential if we are to develop a holistic epistemology of avian vocal communication in both sexes, across ecological contexts and taxonomic divides.

Heterospecific eavesdropping on an anti-parasitic referential alarm call

Referential alarm calls occur across taxa to warn of specific predator types. However, referential calls may also denote other types of dangers. Yellow warblers (Setophaga petechia) produce “seet” calls specifically to warn conspecifics of obligate brood parasitic brown-headed cowbirds (Molothrus ater), which lay their eggs in the warblers’ and other species’ nests. Sympatric hosts of cowbirds that do not have referential alarm calls may eavesdrop on the yellow warbler’s seet call as a warning system for brood parasites. Using playback presentations, we found that red-winged blackbirds (Agelaius phoeniceus) eavesdrop on seet calls of yellow warblers, and respond as much to seet calls as to cowbird chatters and predator calls. Red-winged blackbirds appear to eavesdrop on seets as warning system to boost frontline defenses on their territories, although they do not seem to perceive the warblers’ seets as a cue for parasitism per se, but rather for general danger to the nest.

Lawson et al. study the response of the redwinged blackbirds to referential alarm calls in other bird species. They show that the blackbirds eavesdrop on heterospecific warning calls of the yellow warbler related to the nest parasites and respond by boosting their defenses to general danger on their territories.

Sex-specific defence behaviour against brood parasitism in a host with female-only incubation

Nest protection against intruders is an indispensable component of avian parental care. In species with biparental care, both mates should evolve nest defence behaviour to increase their reproductive success. In most host-parasite systems, host females are predicted to have more important roles in nest defence against brood parasites, because they typically are primarily responsible for clutch incubation. Male antiparasitic behaviour, on the other hand, is often underestimated or even not considered at all. Here we investigated sex-specific roles in four aspects of great reed warbler (Acrocephalus arundinaceus) nest defence against a brood parasite-the cuckoo (Cuculus canorus), namely (1) mobbing, (2) nest attendance/guarding, (3) nest checking and (4) egg ejection. Using dummy experiments, simulating brood parasitism and by video-monitoring of host nests we found that males took the key roles in cuckoo mobbing and nest guarding, while females were responsible for nest checking and egg ejection behaviours. Such partitioning of parental roles may provide a comprehensive clutch protection against brood parasitism.

Experimental evidence for innate predator recognition in the Seychelles warbler

Nest predation is a major determinant of fitness in birds and costly nest defence behaviours have evolved in order to reduce nest predation. Some avian studies have suggested that predator recognition is innate whereas others have stressed the importance of learning. However, none of these studies controlled for the genetic origin of the populations investigated and the effect of unfamiliarity with the predator. Here we determined whether experience with a nest predator is a prerequisite for nest defence by comparing predator recognition responses between two isolated but genetically similar Seychelles warbler (Acrocephalus sechellensis) populations, only one of which had experience of the egg predating Seychelles fody (Foudia sechellarum). Individuals in the predator-free population significantly reduced nest guarding compared to individuals in the population with the predator, which indicates that this behaviour was adjusted to the presence of nest predators. However, recognition responses (measured as both alarm call and attack rates) towards a mounted model of the fody were equally strong in both populations and significantly higher than the responses towards either a mounted familiar non-predator and a mounted, novel, non-predator bird species. Responses did not differ with a warbler's age and experience with the egg predator, indicating that predator recognition is innate.

Influence of experimentally elevated testosterone on nest defence in dark-eyed juncos

Testosterone affects the allocation of reproductive effort in male birds. Elevated testosterone causes male dark-eyed juncos, Junco hyemalis, to decrease care of dependant offspring, but this generalization is based largely on reduced provisioning rates by males treated with testosterone. Therefore, we used a predator model to explore the relationship between testosterone and nest defence, a more immediate measure of male parental effort. Control males (C-males) were more likely to respond within 10 min to a mount of an eastern chipmunk, Tamias striatus, placed at the nest than were testosterone-treated males (T-males). However, among males that did respond within 10 min, T-males initiated nest defence as fast as C-males and defended the nest with equal intensity. Females initiated nest defence more rapidly and struck the mount more often than their mates, regardless of the male's treatment. Overall, the decreased likelihood of T-males being present for nest defence (perhaps mediated by their large home ranges) may increase predation rates at their nests and represent an important cost of elevated testosterone levels. Copyright 1998 The Association for the Study of Animal Behaviour

Host responses to cowbirds near the nest: cues for recognition

Two cues (bill shape and vocalizations) that yellow warblers, Dendroica petechiamay use to recognize brown-headed cowbirds, Molothrus aterwere examined experimentally. Female yellow warblers responded more intensely to a control cowbird than one with a longer, thinner bill, which suggests that bill shape was an important cue in recognition. Responses involved close approaches, 'seet' alarm calls and sitting in the nest. Female yellow warblers distinguished between vocalizations of male and female cowbirds and those of a song sparrow, Melospiza melodia (control). They responded most intensely to female cowbird 'chatter calls' and least intensely to the sparrow song; responses to male cowbird 'perched song' were intermediate in intensity. The results suggest that female yellow warblers recognize cowbirds on the basis of bill shape and vocalizations. Furthermore, these results, coupled with previous findings, suggest that female yellow warblers distinguish between male and female cowbirds by vocalizations but not appearance.