Male sexually selected traits of the barn swallow Hirundo rustica gutturalis in China
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The barn swallow (Hirundo rustica) is represented by six subspecies and has two well-studied ornamental traits, tail length and ventral plumage colour, which vary geographically among subspecies. Sexual selection on these traits has been suggested to drive speciation. The European subspecies rustica has pale ventral feathers and long tail streamers, and females prefer males with longer tail streamers. The North American erythrogaster has shorter tails and red ventral plumage and their females use redness of ventral plumage as a mate choice cue. In the Middle East, the subspecies transitiva bears long tail streamers and red ventral feathers, both of which have been suggested to show male attractiveness. The Asian subspecies gutturalis has a pale belly with short but dimorphic tails. Studies in Japanese populations have suggested that the white spot on the tail feathers and throat patch are sexually selected in males, but this explanation leaves the dimorphism of tail streamers unexplained. To further investigate the sexually selected traits of gutturalis, especially the role tail streamers might play, I studied a population of the barn swallow gutturalis in China between 2013 and 2015, and conducted a partial cross-fostering experiment in 2015. My data indicate that male tail streamers are sexually selected in the gutturalis population in China. Longer-tailed males (with deeper fork tails) were cuckolded less frequently and had a higher reproductive success, they also had mates that invested more in parental care and had a higher total peak body mass of offspring. Both male and female body condition (body mass and tarsus length) influenced offspring growth. Males with larger body mass initiated breeding earlier and their offspring had a larger body mass on day seven after hatching and grew faster as determined by a cross-fostering experiment. Larger females reared offspring that grew faster and reached a higher peak body mass both in original nests and nests with cross-fostered nestlings. Based on feeding rate observations and the cross-fostering experiment, it seems that females obtain indirect benefits rather than direct benefits from mating with more attractive males.
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