So you're an unattractive fairy wren...

An "attractive" male red-backed fairy wren, Malurus melanocephalus. Photo by Greg Miles.

To start we have to point out that you are still pretty darn cute. This week's paper is by Jenélle Dowling and Mike Webster here at Cornell, published in Biology Letters. Red-backed fairy wrens live along the northern coast of Australia. The older adult males have the striking black and red plumage you see in the photo above, while the females and younger males are tawny brown. One of the cool things about this species is that males turn red-black at different ages, so there are brown males that are the same age as red-black males. Females prefer red-black males to brown males, but brown males still form pairs and raise offspring. Fairy wrens, however, have quite a bit of so-called "extra-pair paternity" or EPP. Male fairly wrens sneak off to mate with other females, but when they do they leave their own female un-guarded to mate with somebody else. Males therefore face a trade-off between gaining paternity elsewhere and losing paternity at home. 

Dowling and Webster predicted that the more attractive red-black males would spend more of their time investing in attempting to get EPP's. In contrast less attractive brown males would spend more time mate-guarding their females at home. Additionally they predicted that the younger red-black males would take an intermediate tactic. When they watched the behavior of fairy wrens in the field this is exactly what they saw. They showed paternity corresponded to behavior in that brown males had more offspring within their mate pair and red-black males had more offspring outside their mate pair. This is a lovely example of how behavioral flexibility can shape the costs and benefits of sexual signals.