Male barn swallows use different resource allocation rules to produce ornamental tail feathers

Abstract

Sexual ornaments compete for resources with other functional traits. Such resource allocation trade-offs should ensure the honesty of sexual ornaments according to the Zahavi’s handicap principle. However, the existence of costly signals could not be enough to guarantee honesty if different individuals invest different proportions of their limited resources in ornaments. Then, a certain level of sexual signaling would correspond to several levels of individual condition. Here, we explore whether there are different resource allocation rules in tail feather ornaments between males within a barn swallow (Hirundo rustica) population and whether these different rules confer different viability to males. We assessed the proportion of resources invested in ornamental feathers compared with other functional feathers moulted and growing during the same period at expenses of the same resources. We found that 1) different males allocate a different proportion of resources to ornamental feathers in relation to functional feathers and this proportion is repeatable between years and 2) male survival likelihood decreased as the proportion of resources allocated to ornamental feathers increased. Survival costs associated with increased investments in ornaments can maintain the sexual signaling system honest at population level but do not preclude the existence of an array of different allocation rules between males. Thus, males with different viability can produce ornamental feathers of the same length. These results show that the relationship between male viability and ornament expression can be less straightforward than considered previously. Key words: barn swallow, honesty, resource allocation, sexual ornaments, tail feathers, tradeoff. [Behav Ecol]