Experimentally altering pre-breeding sex steroids reduces extra-pair paternity in female tree swallows

Testing hormonal responses to real and simulated social challenges in a competitive female bird

Competitive interactions often occur in series; therefore animals may respond to social challenges in ways that prepare them for success in future conflict. Changes in the production of the steroid hormone testosterone (T) are thought to mediate phenotypic responses to competition, but research over the past few decades has yielded mixed results, leading to several potential explanations as to why T does not always elevate following a social challenge. Here, we measured T levels in tree swallows (Tachycineta bicolor), a system in which females compete for limited nesting cavities and female aggression is at least partially mediated by T. We experimentally induced social challenges in two ways: (1) using decoys to simulate territorial intrusions and (2) removing subsets of nesting cavities to increase competition among displaced and territory-holding females. Critically, these experiments occurred pre-laying, when females are physiologically capable of rapidly increasing circulating T levels. However, despite marked aggression in both experiments, T did not elevate following real or simulated social challenges, and in some cases, socially challenged females had lower T levels than controls. Likewise, the degree of aggression was negatively correlated with T levels following a simulated territorial intrusion. Though not in line with the idea that social challenges prompt T elevation in preparation for future challenges, these patterns nevertheless connect T to territorial aggression in females. Coupled with past work showing that T promotes aggression, these results suggest that T may act rapidly to allow animals to adaptively respond to the urgent demands of a competitive event.

Does experimentally altered plumage brightness influence extra-pair mating success in female Tree Swallows (Tachycineta bicolor)?

Recent empirical evidence suggests that ornamental traits displayed by female birds may reflect aspects of their quality, and function during competitive interactions and (or) social mate attraction; however, less is known about how such traits influence extra-pair paternity. In Tree Swallows (Tachycineta bicolor (Vieillot, 1808)), plumage brightness of females signals their quality and may be related to extra-pair paternity if it enables them to invade the territories of other females to seek extra-pair copulations and (or) if potential extra-pair mates perceive their plumage brightness as attractive. Therefore, to examine whether the plumage brightness displayed by females influence rates of extra-pair paternity and the number of sires per brood, we experimentally enhanced and reduced the plumage brightness of females relative to controls. Our results showed that plumage brightness treatment of the female did not influence the number of extra-pair offspring in nests or the likelihood of a brood containing extra-pair offspring. Additionally, the number of extra-pair males siring offspring within the broods of females did not differ by plumage brightness treatment. Although extra-pair paternity has been shown to be beneficial for female Tree Swallows, our results suggest that plumage brightness of females does not influence their ability to engage in extra-pair mating.

Effect of testosterone blockers on male aggression, song and parental care in an arctic passerine, the Lapland longspur (Calcarius lapponicus)

In many passerine birds, testosterone stimulates song and aggression but inhibits paternal care, but few studies have explored whether such effects can be reversed with testosterone blockers. We explored the effect of testosterone blockers on song, aggression and paternal care of Lapland longspurs (Calcarius lapponicus), an arctic passerine with a short breeding season. Twenty-one "blocker males" received implants containing an androgen receptor blocker and an aromatase inhibitor, compared to 27 control males with empty or no implants. Song, aggression and other behaviors were evaluated with simulated territorial intrusions (STI) during mate-guarding, and with focal observations (without STI) during mate-guarding and incubation. Nests were monitored and nestlings weighed as an indirect measure of paternal care. During STI, blocker males exhibited similar song rates, significantly lower aggression, and were significantly less likely to be found on territory than control males. Focal observations revealed no differences in spontaneous song, aggression, foraging, preening, or flight activity. Blocker males' nestlings had greater body mass on day 5 after hatching, but this difference disappeared by fledging, and both groups fledged similar numbers of young. Two blocker males exhibited unusual paternal care: incubation and brooding of young, or feeding of nestlings at another male's nest. In sum, testosterone blockers affected aggression but not song, contrasting with results from previously published testosterone implant studies. Effects on paternal care were concordant with testosterone implant studies. These patterns may be related to rapid behavioral changes characteristic of the short breeding season of the Arctic.