True deception during extra-pair courtship feeding: cheating whiskered tern Chlidonias hybrida females perform better

Males of many bird species feed their mates during the pre-incubation period. The food provisioned by males during these courtship feedings (CFs) represents the key source of energy for the female during egg formation. Non-pair males may trade food for extra pair copulations (EPC) with females during extra pair courtship feeding (EPCF), while females may trade copulations for food with non-pair males to obtain additional resources. Because EPCs can be costly to the females, they are expected to behave in ways that will deceive non-pair males to obtain additional resources at no cost to themselves. We investigated EPCFs in whiskered terns Chlidonias hybrida breeding in food-rich conditions, on carp ponds in southern Poland. Almost all CFs (n=2751) took place during the female's fertile period and peaked just before clutch initiation. 10% of all CFs were performed by non-pair males. Females tried to obtain food from the non-pair male during 39% of EPCFs, by swindling (the female solicits a non-social male for copulation and tries to swindle food with no cloacal contact) or by snatching (the female tries to take the gift without engaging in copulation). In the remaining 61% of EPCFs, females did not react or chased the visiting male away. The probability of a female's obtaining food during EPCF was much higher (0.69, 95% CI: 0.47-0.85) if she swindled rather than snatched (0.08, 95% CI: 0.02-0.22). Only 0.7% of EPCFs were followed by EPCs. The high availability of food in the study area allows males to perform frequent EPCFs, despite the very low probability of obtaining EPCs. This is the first time that true deception' during EPCFs has been reported in birds: swindling females obtain food from non-pair males at no immediate detectable cost to themselves.