Résumé
In many socially monogamous animals, females engage in extrapair
copulation (EPC), causing some broods to contain both within‐pair and
extrapair young (EPY). The proportion of all young that are EPY varies
across populations and species. Because an EPC that does not result in EPY
leaves no forensic trace, this variation in the proportion of EPY reflects
both variation in the tendency to engage in EPC and variation in the
extrapair fertilization (EPF) process across populations and species. We
analyzed data on the distribution of EPY in broods of four passerines
(blue tit, great tit, collared flycatcher, and pied flycatcher), with
18,564 genotyped nestlings from 2,346 broods in two to nine populations
per species. Our Bayesian modeling approach estimated the underlying
probability function of EPC (assumed to be a Poisson function) and
conditional binomial EPF probability. We used an information theoretical
approach to show that the expected distribution of EPC per female varies
across populations but that EPF probabilities vary on the above‐species
level (tits vs. flycatchers). Hence, for these four passerines, our model
suggests that the probability of an EPC mainly is determined by ecological
(population‐specific) conditions, whereas EPF probabilities reflect
processes that are fixed above the species level.