In socially monogamous birds, extrapair mating is a common strategy amongst breeding individuals. By mating with multiple females, males may reap fitness benefits by siring more offspring, but this is contingent on the security of paternity with his social partner. Males, therefore, face a trade-off between investments in seeking extra-pair copulations and mate-guarding, the balance of which is likely an outcome of complex social interactions involving the availability of other male competitors and the fidelity of his social partner.

In addition to the social scene, abiotic factors such as weather may influence how males budget their breeding activities. A number of empirical studies have suggested a relationship between weather conditions and extrapair offspring, but few have investigated actual changes in mating/guarding behaviour. The role of the environment in male strategy may provide clues to ecological conundrums such as variations in the occurrence of extrapair offspring.

This study aims to identify the environmental and social contexts involved in mating/guarding Barn swallow males. The effects of local weather conditions on male guarding effort, female evasion of guarding, and male-male competition, were examined. The relative importance of each context in male decision-making is discussed.