Alert Calls of Male Red-Winged Blackbirds: Do Females Listen?

Male red-winged blackbirds (Agelaius phoeniceus) have an unusual vocal alert system that operates throughout the breeding season. Territory owners give alert calls repetitively during most of their activities and transmit information about potential predators when they switch call types. Males are known to listen to each other's alert calls, presumably benefiting by receiving anti-predator information. Results of three separate tests supported the hypothesis that nesting female redwings also listen to and benefit from male alert calls. 1) Average call rates of males monitored through the breeding season increased significantly when their first mates started to incubate. Alerting information about predators may be especially important to females during this period because their ability to detect approaching predators is reduced while they are incubating in dense vegetation. 2) Nesting females gave different responses to playback of repetitive calls without call type changes ("background" calls) and with call type changes (which mimic a male changing call types after detecting a predator). Females were more vocal during broadcast of the tape containing call type changes, and, in some cases, they were also more likely to remain near their nests. 3) Females were more vigilant when their calling mates were absent from their territories, and less vigilant when their mates were present, suggesting that territory owners act as vocal sentinels.

Affiliations: 1: (Department of Zoology, NJ-15, University of Washington, Seattle, WA 98195, U.S.A.