Breeding ecology of northern pintails : nesting ecology, nest-site selection, nutrient reserve use and brood ecology
Guyn, Karla Lee
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To meet nutritional demands of egg laying, female pintails obtain nutrients from either exogenous or endogenous sources. I examined use of nutrient reserves during egg formation in pintails and tested whether reserves regulated clutch size. I found that females relied heavily on fat reserves during egg laying, but found no evidence that fat, or protein, proximately limited clutch size. I tested whether nest-site selection patterns were based on vegetative features, female characteristics, or whether site selection was based on nest microclimate. Pintail nests were characterized by less short grass, more shrub cover and were more frequently found in depression than random sites; unsuccessful nests were closer to shrubs than successful ones. The microclimate of nests differed from that of random sites, nests being about 2°C cooler on average than random sites during daylight hours. Furthermore, 30-minute mean temperatures exceeded the upper lethal limit for embryonic development more often at random sites than nest-sites. I evaluated variation in nesting effort and success of female pintails breeding in prairie habitats. I found no relationship between egg size and clutch size, or evidence from one year to the next of a trade-off between current and future investment in eggs. However, greater investment in initial clutches led to longer delays in laying replacement clutches the same year. Therefore, because delays in renesting are costly (late-nesting females produce fewer offspring), females must contend with a tradeoff between maximizing reproductive output in initial clutches versus risk of delayed renesting. I examined brood and duckling survival from radio-marked females and related duckling survival to maternal and environmental attributes. Duckling survival ranged from 42-65% with most duckling mortality occurring during the first 10 days post-hatch. Ducklings in relatively small broods had higher survival than ducklings from larger broods in 2 of 3 years, suggesting a further constraint on clutch size. Thus, trade-offs occurring at several stages likely set limits to clutch size in pintails, illustrating the need for a more comprehensive template in future studies of clutch size determinants in waterfowl and other species.