Effect of Clutch Size on Incubation Persistence in Male Wilson's Phalaropes (Phalaropus tricolor)

ABSTRACr.-The effect of clutch size on incubation persistence of male Wilson's Phalaropes (Phalaropus tricolor) was studied over two breeding seasons in south-central Saskatchewan. The Wilson's Phalarope is a "sex-role-reversed" shorebird in which only males incubate and provide parental care. The breeding season is long enough to allow for renesting but not for the rearing of more than one brood per male. Barring secondary mating strategies, the total annual reproduction of a male is limited by the size of its clutch. Females lay determinate clutches of four eggs. By determining the minimum clutch size necessary for male incubation persistence, we demonstrated that males alter their reproductive effort in response to clutch size, and we identified the minimum potential annual reproduction for males necessary to maintain this unusual mating system. Clutch-size reductions led to an increased frequency of abandonment inversely proportional to the number of eggs remaining in the clutch. The timing of egg loss also was critical. Among males with clutches reduced to one egg or reduced to two eggs early in the incubation cycle, all but one abandoned incubation. Among males with clutches reduced to two eggs late in incubation or three eggs early, an intermediate proportion abandoned incubation. All but one male with three eggs late in incubation or a full, four-egg clutch continued to incubate. Experimentally increasing clutch sizes by two eggs over the normal four-egg clutch led to rapid nest failure. This suggests that the determinate, four-egg clutch of females is not suboptimal for males and that nest parasitism of two eggs by females has little potential benefit as an alternative reproductive tactic. Received 24 February 1992, accepted 22 November 1992.