Paternity assurance responses to first-year and adult male territorial intrusions in a courtship-feeding raptor

Extrapair paternity occurs in many bird species and males use mate guarding, aggression or frequent copulations to reduce the risk of losing paternity. Because paternity assurance behaviour is likely to be costly, it should be adjusted to varying levels of perceived risk. We evaluated experimentally the adjustment of paternity assurance behaviour to risk in a raptor in which courtship feeding occurs: the Montagu's harrier, Circus pygargus. We used decoys with or without prey to simulate territorial intrusions by adult or first-year males. Inexperienced first-year males should represent a lesser risk than adult males and decoys with prey should represent a greater risk than those without prey, because courtship food could be traded for extrapair copulations. Aggression was less intense towards first-year than adult males during the female's fertile period. However, when decoys were presented with prey, aggression was similar towards first-years and adults. Males increased their short-term copulation rate in response to male decoys, particularly those presented with prey. Males also increased copulation duration, but only in response to adult male decoys. Our results show that male Montagu's harriers can fine-tune their paternity assurance behaviour to varying levels of risk, in this case the type of territorial intrusion. We also discuss the implications of our findings for the adaptive significance of delayed maturation in plumage colour in this species.