Smack in the middle of Red-winged Blackbird territory in the northwest corner of the park sat this blackbird (right) without red on its wings. Was this a Brewer’s Blackbird, a relative with wings that are a uniform glossy black? Or was it an exceptional Red-winged Blackbird who happened to be born without the scarlet epaulets that give this species its common name? Although the birds are capable of hiding their wing colors, this individual preened and fluffed and turned while I was watching it, leaving no doubt that it was the same color all over.
There are Brewer’s Blackbirds in the park, but I’ve only seen them in this area very occasionally, and only in winter, before the Red-winged Blackbirds arrive en masse. Usually the Brewer’s hang out in the southeast corner of the park, diagonally opposite the red-winged variety. The red-winged birds are famously territorial and will drive off any other species. I’ve not found information in the literature about interbreeding between the Brewer’s and the Red-winged, but this seems unlikely as their mating habits are quite different. The Brewer’s are monogamous, with each pair jointly raising two broods per season. The Red-winged are quite promiscuous; although pairs form socially, a male may mate with a dozen females and the females may respond in kind, resulting in broods of mixed paternity.
A study of blackbird evolution concluded that the original blackbird males were all black, similar to the Brewer’s, but that over time, birds who developed coloration on the shoulder (epaulets) found more favor with the females and so came to multiply and prevail over the plain. The authors cite other studies where experimenters removed or painted over the colored epaulets, with the result that the bird lost its territory and thus its chance to reproduce.