Saturday, July 30, 2011

Black-headed Grosbeak

Erika and I found this Black-headed Grosbeak singing in cottonwoods behind a Wyoming rest stop.  This grosbeak is the western counterpart of the eastern Rose-breasted Grosbeak. The two species hybridize where their ranges overlap in the Great Plains.

Males are monogamous and share nesting duties equally with their mates.  Both sexes sing, the males to establish territories, the females to retain communication with their mate and young.  According to Ortega and Hill (2010), "Occasionally, females sing full "male song, apparently to deceive mates about the presence of intruders and force greater nest attentiveness."

The male in this photograph is probably in its first breeding season.  Males do not molt into adult plumage until their second breeding season and this bird retains streaky flanks and does not have a completely black head.  Only first year birds that closely resemble the older males are able to hold territories and breed.  Males that look more like females suffer less aggression from breeding males.  Finally, bright plumage is correlated with testes size (Ortega and Hill 2010).

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