The yellow-headed blackbird, Xanthocepalus xanthocephalus, perched near a marsh. Yellow-heads, like their relative, the redwinged blackbird, have a polygynous mating system, in which a male territory-holder may have more than one female mating within his territorial boundaries.

Until recently, scientists assumed that the progeny of the females within a male's territory were all his. However, it is now recognized that females of many bird species participate in extra-pair copulations (EPC's) and that as a result a significant number of the nestlings in a male's territory may not be his.

Why might the male be so brightly colored? Here are three hypotheses:

• The color may help to attract females (this would make the color a sexually-selected trait). If the color attracts predators, it would also be a handicap.
• The color may signal to females that the male is relatively healthy. Unhealthy males may, due to poor nutrition, parasites, or disease, be unable to produce the bright colors.
• The color may warn other males of the presence of a territorial male.

A territorial male yellow-headed blackbird in a marsh near Boulder, Colorado. A female, lacking the bright coloration of the male, is visible in the background.

Beletsky and Orians (1994) found that 42.9% of the males in their study population changed territory sites between years, and that over time, 60% of the males changed sites.

These site changes did not seem to improve territory quality or reproductive success for the birds. Perhaps yellow-heads simply choose new sites yearly, based on their assessment of conditions. Another possibility is that the declining habitat quality in the study area caused birds to shift territory sites.