The adaptive significance of male subadult plumage in purple martins: plumage dyeing experiments

Abstract

I tested two hypotheses for the adaptive significance of subadult plumage in male purple martins (Progne subis): the female mimicry and subordinance signaling hypotheses. Subadult males were at a competitive disadvantage in obtaining territories, as they arrived later in the spring than adult males. Contrary to the predictions of both hypotheses, adult male territory owners were not less aggressive toward subadult male than adult male intruders. The subadult plumage was not effective in mimicking females, as adult male owners were significantly more aggressive toward subadult male than female intruders. Summer adaptation hypotheses predict that young males in subadult plumage are more successful in acquiring territories and mates than they would be with an adult plumage. I tested this prediction by dyeing the plumage of "floater" subadult males to mimic the appearance of adult males. In 13/17 paired experiments, dyed subadults obtained territories before control subadults. There was no difference in the time it took dyed and control males to attract a mate after they obtained a territory. These results suggest that the subadult plumage is not an advantage to young males in competing with adult males for breeding resources. In late winter, subadult males were growing mostly femalelike feathers on their underside, suggesting that the subadult plumage is not the result of a molt constraint. The subadult plumage could enhance survival of yearlings in winter roosts if it improves access to good roost sites or reduces the risk of predation.

Delayed plumage maturation increases overwinter survival in North Island robins

“…The female-like plumage may still be adaptive at this stage. Stutchbury (1991) also noted that it could be maladaptive for birds to have a mixed plumage, and suggested this is why sub-adult male purple martins produce female-like plumage in late winter even though her experiments suggested it would be advantageous for them to have a definitive male plumage at this stage.…”

“…The 'winter adaptation hypothesis' can therefore potentially explain DPM through the first breeding season even if sub-adult plumage is maladaptive while breeding. This could potentially explain results such as the finding of Stutchbury (1991) that DPM apparently disadvantaged sub-adult purple martins. It is also possible that DPM may occur for non-adaptive reasons (Stutchbury 1991;Landmann & Kollinsky 1995), so the winter adaptation hypothesis should not be invoked uncritically in the absence of evidence for summer adaptation hypotheses.…”

Iridescence: a functional perspective

“…In purple martins (Progne subis), adult males have blue-black iridescent plumage, whereas subadults have a mixture of blue-black and brown feathers (Stutchbury 1991). Subadult males that were experimentally dyed black to mimic adult males acquired breeding territories faster than control subadults.…”

Condition-dependent control of paternity by female purple martins: implications for coloniality

“…Male purple martins exhibit delayed plumage maturation: males age two years or more attain the definitive purple whereas yearlings retain a mainly brown and white female-like juvenile plumage, despite undergoing a complete molt (Stutchbury 1991a). observed that older males could increase their fitness by recruiting young males and cuckolding them, and suggested that the benefits old males accrued from EPCs select for coloniality.…”