How western meadowlarks respond to simulated intrusions by unmated females

Summary

Western meadowlarks (Sturnella neglecta) were played back female calls at various stages of their breeding cycles, to examine how residents might respond to unmated females. Females reacted to intruders with aggressive displays and other responses that varied in intensity (Table 1). Males reacted not only to playback, but also to their mates' responses, often intervening in them. The responses of both males and females, however, were strongly affected by the stage of the breeding cycle at which an intrusion was simulated (Table 3). Females responded to playback most strongly before they began incubating eggs; thereafter, the strength of their responses declined rapidly. Males' also responded less strongly at later stages of their mate's nesting cycle, but the strength of their responses declined less rapidly than did the females'. When the reactions evoked from mated individuals were compared, females responded at least as strongly as their mates, until after their young had fledged (Fig. 3). These seasonal changes in the intensity of females' responses suggest that decisions about how strongly to respond take account of the costs of responding to an intruder, as well as its benefits. Our results suggest that interactions among mated and unmated females may affect the timing of bigamy and, in some cases, its incidence as well.