One of the costs to female birds of mating polygynously is having to share the parental care provided by their mate. Nest defence, however, is one form of paternal care that females mated to the same male should be able to share. In this study there was no difference in the defence of primary and secondary nests by male red-winged blackbirds, Agelaiusphoeniceus. Nests that were provisioned by males were not defended more aggressively than nests that were not provisioned. Nests that were successful had been defended more aggressively by males than those that failed, indicating that male nest defence is valuable to females. These results suggest that male nest defence is entirely shareable, in contrast to a recent study that showed males preferentially defended nests of primary females (Knight & Temple, Condor, 1988, 90, 193- 200). Although the study design used here more realistically reflects natural nest defence (males had to defend one nest at a time rather than having to choose which of two nests to defend), it does not explain why males preferentially defended certain nests when forced to make a choice but not when defending nests one at a time.
Data were collected from May to July 1988 as part of a broader study of the reproductive biology of red-winged blackbirds being conducted at the Queen's University Biological Station in eastern Ontario. Study sites consisted of 7 marshes located within 10 km of the Biological Station.