The relationship between plumage coloration and aggression in female tree swallows

Intrasexual competition is an important selective force that can favor the evolution of honest signals of fighting ability.  Research has focused predominantly on male birds, but many female birds also possess plumage ornaments that could mediate the outcome of competitive interactions.  We examined the relationship between blue and white structural coloration and aggression in female tree swallows (Tachycineta bicolor).  Tree swallows are secondary cavity nesters and females show delayed plumage maturation which may be related to intense competition for nest sites.  We compared plumage reflectance of second-year (SY) and after-second year (ASY) females, and within-individual changes in plumage reflectance of ASY females in two successive years.  We assessed aggression by placing a caged SY female 2 m from the nest box of an ASY female and quantified the ASY female’s response in relation to her own plumage coloration.  We found substantial differences in plumage reflectance of SY and ASY females, but found that ASY females became greener with duller white coloration in the second year.  This may be due to poor weather that made reproduction particularly costly in the first year of the study and suggests coloration is influenced by condition.  We found no relationship between dorsal coloration and aggressiveness.  Rather, brighter white females spent more time on their nest box and within 2 m of the intruder than females with dull breasts.  Our findings suggest that brighter white ASY females may perceive a SY female as less of a threat or that there may be a trade-off associated with aggression and parental care that is related to white brightness.