Extra-pair paternity in Swainson's Hawks

Although passerines have been relatively well studied and many species found to exhibit relatively high rates of extra-pair paternity (EPP), less is known about the frequency of EPP in other avian taxa, including raptors. From 2008 to 2010, we examined the frequency of EPP in a population of Swainson's Hawks (Buteo swainsoni) in Butte Valley, California. We examined paternity of 56 nestlings from 19 pairs and 27 broods and found that only three nestlings (5%) in two (7%) broods were the result of extrapair fertilizations. This relatively low frequency of EPP may be the result of mechanisms that reduce the likelihood of extra-pair fertilization (e.g., mate guarding and frequent copulation), or could result from females limiting EPP to assure paternity of the social male and ensure paternal investment in offspring.