Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

White-throated sparrows ( Zonotrichia albicollis ) exhibit two color morphs that breed disassortatively with each other, creating a rare four-sex system. This polymorphism, linked to an inversion on Chromosome 2, also results in two distinct behavior profiles and alternate reproductive strategies. Males settle in territories with characteristics that optimize their reproductive strategy. Overall, the objective of this project was to investigate nest success over a long time-period (18 years). I first sought to create an interpolated risk map for nest failure, a major cause of decreased reproductive success in both sexes. High-density areas, associated with white males, contained more failed nests than successful ones, and one of these areas also showed a spike in nesting risk. Conversely, the only consistently low-risk area was located near a beaver pond, an area typically associated with tan males. White males are likely not suffering diminished reproductive success from nesting in high-risk areas, because they tend to seek extra-pair copulations with neighboring females, thus scattering paternity among several nests. Tan females in these high-risk areas, however, have no such insurance in the case of nest failure. They are likely benefiting from increased genetic diversity among their young from extra-pair paternity, which might increase the chances of juveniles surviving to the next breeding season. Second, I sought to analyze habitat effects on nesting success at several spatial scales. I found that neither territory type (broad scale) nor habitat type (medium scale) had an effect on nest fate. Nests that are farther away from trees tend to be more successful, likely related to avian predators that might perch in those trees. Nest cover did not affect the fate of the nest, although a nests height above the ground seemed to indicate a pattern counter to common ecological dynamics. Further investigation revealed that nest height is more likely a response to increased predation pressure later in the season, not the cause of the increased failure rate; both nest height and success are strongly correlated with clutch synchrony and timing. Taken together, these results reveal additional trade-offs between the four white-throated sparrow ‘sexes, where the two morphs seem to remain at reproductive equilibrium. They also indicate that females use behavioral plasticity to maximize their nest success potential in a changeable social and ecological environment.

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