Male Red-winged Blackbirds (Agelaius phoeniceus) are known to give alarm calls in response to the approach of a predator, and to encode information about the level of threat in their calling behavior. To determine whether such sentinel males alert females, we conducted a simple field experiment in which we measured the distances at which incubating females flushed from their nests in response to the approach of a human observer. Using a matched-pairs design, we measured flushing distances with a sentinel male present (mean 19.8 m), and when the same male was absent from his territory (mean 10.4 m). Female Red-winged Blackbirds flushed from their nests at significantly greater distances when males were present than when males were absent. These results and those of other studies support the existence of a “predator early warning system” in the Red-winged Blackbird.
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Vol. 72 • No. 1