Ken Yasukawa

Intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds: an experimental approach

We conducted experiments on the intersexual and intrasexual consequences of epaulet colour in male red-winged blackbirds, Agelaius phoeniceus . In the female choice experiment, we gave captive females a choice between males with normal or dulled epaulets, and between males with normal or reddened epaulets. Females tended to associate more with normal males than with dulled males, but were equally likely to perform precopulatory displays to the two types of males. Females associated significantly more with redder-than-normal males and were more likely to perform precopulatory displays to reddened males. In the male–male competition experiment, we presented free-living territorial males with a male model to which we could attach wings with dulled, normal or reddened epaulets. Presence of the model with reddened epaulets resulted in significant increases in display rates by territory owners, but the increase in display rates was probably in response to males other than the territory owner flying through and trespassing rather than to the reddened epaulet model per se. Normal epaulets produced a significant decrease in song rates and approach distances of owners. Responses of territorial and other males did not differ significantly during normal and dulled epaulet presentations. These results support the hypothesis that epaulets of male red-winged blackbirds are salient inter- and intrasexual signals: unusually red epaulets were attractive to females and attracted more attention and aggression from male conspecifics than normal epaulets. Epaulet colour may thus be an example of intrasexual selection opposing intersexual selection.

THE “PREDATOR EARLY WARNING SYSTEM” OF RED-WINGED BLACKBIRDS

Abstract 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.

Epaulet Color and Sexual Selection in the Red-Winged Blackbird: A Field Experiment

Abstract. The epaulets of male Red-winged Blackbirds (Agelaius phoeniceus) function in both intersexual and intrasexual contexts, but there is little evidence that they covary with reproductive success in this well-studied species. We used path analysis of male/territory traits, mating success, and reproductive success of unmanipulated males to estimate current directional selection. Territory size had a positive effect on number of within-pair mates. Number of extra-pair mates had a positive effect on number of extra-pair fledglings, and number of within-pair and extra-pair fledglings had positive effects on total number of fledglings. We also reddened epaulets of free-living territorial males to determine whether manipulated and control males differ in territorial behavior, mating success, or reproductive success. Compared with control males, males with reddened epaulets incurred elevated rates of trespassing and territorial challenges, lost their territories more often, and were unable to produce extra-pair offspring. Despite these differences, however, the realized reproductive success of experimental and control males did not differ significantly, perhaps because males with reddened epaulets devoted more time to anti-predator vigilance and were more aggressive toward a simulated predator than were control males. An apparent lack of current sexual selection on epaulet color may be a form of counter-balancing sexual selection in which male aggression against redder epaulets opposes female preference for redder epaulets, or it might be the result of males compensating for lost extra-pair fertilizations by increasing their parental care.

Yellow-billed Cuckoo Hatched and Fed by a Red-winged Blackbird

Abstract I observed a Red-winged Blackbird (Agelaius phoeniceus) nest in southcentral Wisconsin, USA that received a Yellow-billed Cuckoo (Coccyzus americanus) egg. The cuckoo egg was laid after the blackbird clutch was complete and the female had begun incubation. The parasitic egg was considerably different in size and color, but was accepted by the female Red-winged Blackbird. The female, but not the male, also cared for the nestling, which was strikingly different in plumage, gape markings, and begging behavior, and which grew despite its 3–4 day hatching delay relative to its nest mates. Rapid development and exaggerated begging behavior of the Yellow-billed Cuckoo nestling may enable a parasitic cuckoo nestling to compete successfully with host nestlings for food.