Sexual Identity Issues in Male Red-backed Fairy-Wrens
By Gregory Gough
There are 3 distinct types of male red-backed fairy-wrens (a bird that lives in northern Australia): brightly-colored breeders, dull-colored breeders, and dull-colored helpers.
The brightly-colored males are a deep jet black with bright crimson patches. The dull-colored breeders are brown and white, like the females, with a black bill (like brightly-colored males). The helpers are also colored like the females and have a pale bill (like females). Helper males stay on the territory where they were raised and assist the resident pair in raising young.
These 3 types of males occur in young birds. Young males that are the most robust and healthy typically molt into the brightly-colored plumage. Less healthy males retain a female-like plumage and the least aggressive males became the helpers.
Even though the dull-colored males look like females, tests in the aviary confirmed that the brightly-colored males were not deceived but were not as aggressive to them as they were to other brightly-colored males.
So, the dull coloration of some males appears to give them respite from brightly-colored male bullies.
There does not appear to be a cost to having a brightly-colored plumage as all 3 types of males had similar lifespans.
Females appear to prefer brightly-colored males when cheating on their mate so if a brightly-colored male vacates a territory, dull-colored males will change their bill or plumage color to move up on the male social ladder.
This article summarizes the information in this publication:
Karubian, Jordan, Sillett, Terence Scott and Webster, Michael S. 2008. The effects of delayed plumage maturation on aggression and survival in male red-backed fairy-wrens. Behavioral Ecology, 3: 508-516.
The occurrence of multiple phenotypes within a sex of a single species has long puzzled behavioral ecologists. Male red-backed fairy-wrens Malurus melanocephalus exhibit 3 behaviorally distinct types in their first breeding season: breed in bright nuptial plumage, breed in dull plumage, or remain as an unpaired auxiliary (helper) with dull plumage. The retention of dull plumage by auxiliaries and dull breeders is an example of delayed plumage maturation (DPM), a widespread phenomenon in birds whose costs and benefits are not well understood. At a mechanistic level, DPM might allow dull males either to deceptively mimic females (female mimicry hypothesis) or to honestly signal their subordinate status (status-signaling hypothesis). DPM might function via either mechanism to provide ultimate benefits relative to developing nuptial plumage by increasing reproductive success, survival, or both. In this study, we tested the hypothesis that DPM is related to increased male survival in the red-backed fairy-wren via either female mimicry or status signaling. Aviary-based experiments revealed that dull males were perceived as male, which is consistent with the status-signaling hypothesis but contradicts the female mimicry hypothesis. Further aviary and field-based experiments also revealed that dull males were socially subordinate to bright males and received less aggression than bright males, further evidence for status signaling. However, male survival was not related to plumage coloration or breeding status. These findings indicate that male plumage coloration signals social status but that dull plumage does not afford a net survival advantage, perhaps because plumage color is a conditional strategy.
Teachers, Standards of Learning, as they apply to these articles, are available for each state.
