Extrapair paternity, migration, and breeding synchrony in birds

To understand interspecific patterns in the strength of sexual selection, variation in the costs and benefits of exercising mate choice needs to be evaluated. One manifestation of sexual selection in birds is the occurrence of greatly variable levels of extrapair paternity (EPP). A proposed general explanation for this variation is that the benefits to females in seeking extrapair copulations vary in a predictable manner according to the degree of breeding synchrony because females are better able to assess potential extrapair partners when males are simultaneously in breeding condition. This hypothesis predicts a latitudinal trend in EPP because birds tend to breed more synchronously away from the equator. Expanding on previous geographically and taxonomically restricted tropical/temperate comparisons, we used phylogenetically independent standardized linear contrasts to show that this positive relationship persists when all bird species for which EPP estimates currently exist are considered. However, if a third factor covaries with latitude in the same way as breeding synchrony and EPP, this relationship need not be causal. Migration could also account for latitudinal variation in the benefits to females of pursuing EPP, if migration is associated with (1) hasty or (2) inaccurate mate choice, (3) facilitated assessment of male quality through the condition-dependence of arrival time, or (4) increased genetic variance in male quality. We show that migration distance is positively related to the proportion of EPP and that migration can statistically explain the latitudinal trend in EPP, even when confounding factors are simultaneously controlled. Hence, alternative explanations for latitudinal variation in EPP may be feasible, and careful intraspecific tests are needed to assess their relative importance and their implications for geographical variation in life-history evolution.