Decoy presentations as a means to manipulate the risk of extrapair copulation: an experimental study in a semicolonial raptor, the Montagu's harrier (Circus pygargus)

Mate guarding and frequent copulations are two alternative paternity assurance strategies found in birds. In species with intense courtship feeding, like raptors, the “frequent copulation” strategy is expected because male food provisioning conflicts with mate guarding. We evaluated experimentally the paternity assurance behavior of a semicolonial raptor, the Montagu's harrier Circus pygargus, using decoy presentations to simulate territorial intrusions. Breeding pairs were exposed to male and female decoys at different periods during the female's reproductive cycle. Agonistic responses to decoys were intra-sexual, and the timing and intensity of male attacks toward male decoys supported responses related to the risk of extrapair copulation (EPC): Male aggression peaked during the presumed fertile period and almost disappeared after clutch completion. During the fertile period, copulation rate was significantly higher, and copulations lasted longer, during male decoy presentations than during controls. Males also spent more time close to the female during male decoy presentations compared to controls, both during the early prelaying and fertile periods, but not during incubation. In the fertile period, males also increased presence time close to the female in the hour following the removal of the male decoy. Conversely, female decoy presentations had no significant effect on copulatory behavior or male presence time. These results showed that the risk of EPC can be experimentally manipulated by the means of decoy presentations, simulating male territorial intrusions, and that male Montagu's harriers increase their short-term copulation frequency and female surveillance when they perceive themselves at an increased EPC risk.