Honest Signaling in Red-backed Fairy-wrens
Social and Physiological Costs
Why are some male red-backed fairy-wrens dull (left photo) and other males bright (right photo)? My lab has been asking this question for a while now but we haven't quite figured it out. Most males remain in dull plumage during their first breeding season, but some males molt into bright plumage before their first breeding season. By their second year most males take on bright plumage before they breed. We know that females are more attracted to and prefer to mate with males in bright plumage over males in dull plumage, so why wouldn't all males molt into bright plumage for the breeding season?
Initial research looked at the physiological costs of producing and maintaining bright plumage. Bright plumage in this species is testosterone-dependent, meaning in order to produce the bright phenotype males must increase and maintain high levels of circulating testosterone in their blood. But high testosterone levels are known to suppress the immune system and increase metabolism. This led to the prediction that males molting into bright plumage may be of higher quality than males molting into dull plumage because they are better able to deal with the negative effects of high testosterone. However, when this was tested experimentally, males implanted with testosterone were found to be in no worse condition than males implanted with empty implants.
So maybe the physiological cost hypothesis isn't so important after all. But if that's not controlling which males molt into bright plumage earlier than others, what is?
My research is looking at the possible social costs of molting into bright plumage. Social regulation of signals has been well-reported in other animal systems and individuals that attempt to cheat the system, or produce a signal that encodes information about their condition or strength that is not true, often suffer increased aggression from their neighbors. In a similar way, my hypothesis is that if a young male molts into bright plumage he's going to become another competitor for the old bright males nearby to gain mating opportunities with females. Old bright males that see a younger male molting into bright plumage may behave more aggressively towards him to prevent him from molting or to drive him from their social group. It's possible that in order for a young male to molt into bright plumage he must be able to fend off aggressive bright males or be in an social context that lacks bright males.
Come back soon as I begin to answer this question!
Low-amplitude Song in Birds