A Multifactor Study of Clutch Size Determination in the Eastern Kingbird

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Many organisms are currently showing alarming decline in abundance. Among birds, species that build open-cup nests, breed in North America and winter in the New World tropics (Neotropics) are showing the sharpest declines. The reasons for the declines are not known. To understand why populations are suffering it is necessary to document survival patterns of (a) adults, (b) nests, and, (c) independent young. In addition, it is crucial that we understand what factors control and/or limit reproduction. Clutch size (number of eggs/nest) is the primary reproductive variable among birds, and all else being equal, individuals that lay the largest clutches and produce the most young should dominate. It is therefore hard to understand why clutch size in some species of birds is smaller than the most productive brood size (number of young/nest). Understanding this phenomenon is important because reproductive processes have a direct impact on population dynamics, and therefore conservation practices. This project proposes to examine five hypotheses that have been put forth to explain why clutch sizes are often smaller than the most productive brood sizes. The Eastern Kingbird (Tyrannus tyrannus), a long distance migrant that winters in Amazonia, typically lays just three eggs but can raise 4 and sometimes 5 young. Experiments will be conducted to determine if small clutch size in kingbirds is related to (a) low food availability, (b) high predation pressure on nests, (c) trade-offs with parental survival (i.e., raising many young increases adult mortality), (d) trade-offs with offspring survival (i.e., young fledged from large broods are smaller and have lower survival rates), or, (e) unpredictable increases in clutch size due intraspecific brood parasitism (i.e., the addition of eggs to a completed nest by a different female). The results of this study should help to clarify the puzzle of "submaximal" reproduction and also provide specific information on survival, reproductive success and population stability in the Eastern Kingbird.