Inter-specific variation in copulation behaviour among birds is described. The following factors explaining variation in copulation rate are examined: (i) the number of eggs which have to be fertilized (the fertilization hypothesis), (ii) the importance of the pair-bond (the social bond hypothesis), (iii) predation risk during copulation (the predation hypothesis), and (iv) cuckoldry risk (the sperm competition hypothesis). These hypotheses were investigated in preliminary analyses at the generic level using data on copulation behaviour in 131 bird species. The sperm competition hypothesis was supported by several lines of evidence: (i) frequent copulations among genera where males are not able to guard their mates (colonial birds, diurnal birds of prey, and owls), (ii) frequent copulations in polyandrous genera, (iii) copulations inside hole-nests of colonial birds but outside the hole-nests of solitarily nesting genera, and (iv) by the occurrence of forced pair copulations following extra-pair copulation in some species. The predation and social bond hypotheses were not totally dismissed, but there was no evidence that fertilization ability was limited by copulation frequency (i.e. the fertilization hypothesis).