It is not uncommon in the bird world for males to be adorned in bright breeding colors and flashy ornaments while females and juveniles are drabber and more boring. This is not the case in white-necked jacobin hummingbirds from Panama, however.
A recent study published by Cell Press investigates the possible reasons why more than one quarter of female jacobin hummingbirds, along with all juveniles of both sexes, have brightly colored plumage like males.
“One of the ‘aha moments’ of this study was when I realized that all of the juvenile females had showy colors,” says first author Jay Falk, who is now a postdoc at the University of Washington but led the research as a part of the Cornell Lab of Ornithology and Smithsonian Tropical Research Institute.
“For birds that’s really unusual because you usually find that when the males and females are different the juveniles usually look like the adult females, not the adult males, and that’s true almost across the board for birds. It was unusual to find one where the juveniles looked like the males. So it was clear something was at play.”
Male white-necked jacobins have iridescent blue heads and green backs, with white bellies and tails. The juveniles have a similar bright coloration but have an added facial stripe of a buffy color. Females mostly have green backs and heads, with green-grey throats and chests. However, around 20 percent of the adult females are brightly colored like the males.
Falk and his colleagues captured 436 white-necked jacobins over a four-year period, and recorded the plumage details for each individual. The experts found that some females with bright, male-type plumage as juveniles were recorded in subsequent years as adult females with the dull, muted plumage. By contrast, adult females with male-type plumage retained their bright colors in subsequent years.
According to the study authors, this indicates that plumage changes can occur during the time when a female bird is a juvenile but not during its adulthood.
Bright coloration in male birds is usually explained in terms of sexual selection – the process whereby females select males for their colorful appearances. However, in the current study this could not have been the origin of the male coloration pattern because it was present when birds were juveniles – a time in the life cycle when no reproduction takes place.
In order to investigate what other processes may be selecting for male coloration, the researchers placed taxidermy mounts of colorful females and dull females on feeders and observed as other hummingbirds interacted with them. Hummingbirds compete aggressively for nectar, so the researchers recorded incidents of both sexual interaction and aggressive or competitive interaction.
They found that white-necked jacobin males showed a clear sexual preference for the dull colored females, which would explain why the majority of the adult female population had this plumage type. However, the researchers also found that females with male coloration experienced less harassment and aggression both from members of their own species and from other species.
In a further experiment, the researchers recorded visits by tagged hummingbirds to nectar feeders. They found that females with male coloration visited nectar feeders more frequently and stayed for longer durations than did dull-colored females.
The researchers conclude that the male coloration in adult female white-necked jacobins enables these females to avoid harassment while feeding and therefore to feed more successfully than females with dull plumage. Thus, a form of social selection may have given rise to the two different forms of plumage in the adult female population.
The study is published in the journal Current Biology,
By Alison Bosman, Earth.com Staff Writer