EXTRA-PAIR PATERNITY IN POLYGYNOUS TREE SWALLOWS

In tree swallows, Tachycineta bicolor, DNA fingerprinting revealed that polygynous males (N = 7) were twice as likely to have unrelated young in their nests as monogamous males (N = 16; 100% versus 50% of males, respectively). However, on average, the total number of nestlings sired by monogamous and polygynous males was similar because polygynous males had two nests. In nests with mixed paternity monogamous and polygynous males had relatively similar proportions of unrelated young per nest. Polygynous males rarely assisted their secondary mate with feeding nestlings, and, as a consequence, secondary mates of polygynous males produced fewer fledglings than females mated to monogamous males. The net effect of these differences in paternity and fledging success was that polygynous males produced fewer genetically-related fledglings from both nests combined than monogamous males produced from their single nest (0.8 versus 3.0 fledglings). Two possible reasons for the difference in extra-pair paternity between polygynous and monogamous males are discussed. In other populations of tree swallows, secondary females are better able to produce fledglings without male assistance, and in these cases polygyny, even with the cost of extra-pair paternity, may be advantageous to males. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.