American Goldfinches
Carduelis tristis

Goldfinches have sexually dichromatic plumage, yet both female and male bill color changes from grey-brown to bright orange before the breeding season.

Keith Tarvin and I are testing whether female bill coloration signals fighting ability to conspecifics, and whether bill color is evaluated by discriminating males during mate-choice.

Test of female-female status signaling

We tested whether caged females avoided feeding adjacent to female taxidermic models as a function of the model’s bill color, which was experimentally augmented or dulled. We predicted that
augmented-billed females would appear dominant and that females would feed at the feeder protected by the “wimpy” looking female.

We found an overwhelming pattern that 17 of 19 females avoided feeding near competitor females with colorful bills.

This suggest that female carotenoid-based orange bills are evolutionarily maintained through selection for intrasexual status signaling.

This research provides some of the first evidence that carotenoid-based ornaments in females can function as status signals used in contests over non-mate based resources.


Test of mate-choice signaling by females

To test for a male mate-choice function to female bill color, we gave caged males a choice of courting two live females with
experimentally altered bill color.

Males showed no preference for bill color and spent similar amount of time courting augmented-billed and control-billed females.

The lack of effect of female bill color on male behavior suggests that males are not the intended receivers of this signal.


© 2006 Troy Murphy