18 January 2006 - Salineno, Texas. This adult Altamira Oriole, was one of four species of orioles visiting the DeWind's feeders. This species reaches the northern limit of its range in southern Texas, but unlike the previous Audubon's Oriole, it appears to be increasing. Prior to 1950 it was considered rare. It is the largest oriole in the US. Males and females are almost identical. They remain paired throughout the year and are resident. I am fascinated by the fact that migratory orioles show a strong sexual dimorphism while tropical non-migratory species are monomorphic, with both sexes resembling the male. I assume that dimorphism is advantageous to migratory birds who must find new mates each year, while resident species that remain paired throughout the season do not find selective advantage in having males differ from females. But why do both sexes have a bright male-like plumage? In dimorphic species, I assume the bright male plumages are a consequence of sexual selection; females preferring bright (healthy-looking) males. But what possible advantage is there for non-migratory females to be bright looking? I'd be interested in any thoughts on this subject.