Brood size, duckling survival and parental care in canvasbacks

Clutch size in precocial birds is generally thought to be limited by factors acting during egg-laying and incubation. However, hypotheses advanced to explain limits on clutch size are not sufficient to explain observed clutch sizes. I tested if factors acting after hatching can limit clutch size in precocial Canvasbacks, (Aythya valisineria). Clutch size may be limited by post-hatching factors if large broods have higher mortality, or if large broods require more parental care. To examine the effect of brood size on duckling mortality I compared duckling survival in experimentally enlarged and reduced broods. While duckling survival was inversely related to brood size, the relationship was not significant. By modifying an existing model of clutch size determination (Arnold et al. 1987), I demonstrate that even a slight brood size-dependent decline in duckling survival is sufficient to reduce optimal clutch size. To determine if there was a potential for costs of reproduction to vary with brood size, I collected time-activity budgets of females rearing experimentally manipulated broods. If costs of reproduction vary with brood size, females rearing enlarged broods would be expected to spend more time in parental care activities than those rearing small broods. Brood-rearing females did not alter their behaviour with respect to brood size, nest initiation date, or body condition. Duckling survival was unaffected by the amount of time that females spent in parental care or self maintenance activities, suggesting that offspring survival is dependent only on the presence or absence of a brood-rearing hen. The level of parental care that females provide is unlikely have brood size dependent costs, and is thus unlikely to affect optimal clutch size of Canvasbacks.Science, Faculty ofZoology, Department ofGraduat