Sammendrag
In birds with extrapair mating, older males usually have higher fertilization success than younger males. Two hypotheses
can potentially explain this pattern: 1) females prefer older, and often more ornamented males, or 2) older males invest more
in reproduction and fertility than younger males. Here we studied factors associated with age-related male fertilization success in a population of barn swallows Hirundo rustica in Canada. We document that male fertilization success increased
gradually up to a minimum age of four-year old. The age efect was especially strong for the number of extrapair ofspring
obtained and the occurrence of a second brood. The higher fertilization success of older males was also associated with an
early start of breeding in spring. The length of the elongated outermost tail feathers, a postulated male ornament preferred
by females, also increased with age (in both sexes), but it was not a signifcant predictor of male fertilization success within
age classes. Male fertility traits, especially testis size, but also sperm motility and sperm velocity, increased signifcantly
across age groups. Our results suggest that the higher fertilization success by older males is due to their higher reproductive
investments and that their longer tails are an adaptation to early arrival on the breeding grounds.
Extrapair paternity · Life history · Sexual selection · Sperm traits · Tail length · Testes size
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