High frequency of extrapair fertilization in a plural breeding bird, the Mexican jay, revealed by DNA microsatellites

We used tetra-nucleotide microsatellite DNA typing to estimate the frequenc y of extrapair fertilization (EPF) in a plural breeding species, the Mexica n jay, Aphelocoma ultramarina, in Arizona. We found EPF in 32 of 51 complet e broods (63%) and 55 of 139 nestlings (40%) for which the putative father had been identified (one of the highest rates of EPF known for birds). At l east 96.1% of EPF fathers came from within the group. This is by far the hi ghest known within-group EPF rate among socially monogamous, communally rea ring species. Most (70%) males of breeding age (3+ years) had no genetic pa ternity in a given year. Social fathers (i.e. those with nests and mated fe males) rarely obtained EPFs; of 25 social fathers, 23 had young in only one nest and only two had young in two nests by virtue of EPF. Of the 27 males known to be EPF fathers without a nest of their own, none had young in mor e than one nest. Only 7% of EPF fathers had their own broods reaching bandi ng age (day 14), compared with 29.7% of social fathers. The proportion of E PF young was significantly larger in smaller broods. Breeding females in al l age classes were equally likely to have EPF young. (C) 2000 The Associati on for the Study of Animal Behaviour.