Differences in parental contribution among pair types in the polymorphic White-throated Sparrow

We compared relative contributions to nestlings between male and female parents among different pair types in the White-throated Sparrow (Zonotrichia albicollis). The amount of parental contribution between the two main pair types, white-striped (WS) male × tan-striped (TS) female and TS male × WS female, did not differ. However, we found that (a) pairs consisting of TS males and WS females showed biparental care in which the partners made approximately equal investment in feeding nestlings; (b) males of WS male × TS female pair types contributed far less than the female, and far less than a TS male; (c) in WS pairs, males and females contributed about the same as did WS birds in mixed pairings; and (d) secondary TS females nested in territories of already paired WS males, and these females raised young unassisted by a male. We suggest explanations for these differences in contribution to nestlings between the sexes and between morphs, and we discuss optimal reproductive strategies for each sex and morph.