Differential Migration, Molt and Departure Date in White-Throated Sparrows, Zonotrichia albicollis

This study investigated several aspects of the avian life cycle, examining how closely the events of one season are tied to those of another. I investigated differential migration, molt and departure date in White-throated Sparrows, Zonotrichia albicollis. It is known that birds that arrive earlier on their breeding grounds are more successful than later-arriving birds. With that said, there is still considerable unexplained variation among departure dates from wintering grounds. This is particularly interesting when studying differential migrants, in which males and females migrate different distances to their wintering grounds. Differential migration is a well-studied phenomenon; however, the causes have yet to be determined. Until this study, there has only been suggestive evidence that the Whitethroated Sparrow is a differential migrant. Therefore, in the first part of the study I investigated population segregation by sex in this species on the wintering grounds. As predicted, I found that White-throated Sparrows were exhibiting differential migration. This was indicated by a latitudinal cline in sex ratio in the Atlantic flyway resulting in higher proportions of females at more southerly latitudes according to Bird Banding Lab data, museum specimens and field site data. Learning that this species is a differential migrant allowed me to look into one possible cause for this pattern, that there is selective pressure on males to arrive early on breeding grounds, and therefore they cannot migrate as far as females. There has been only suggestive evidence for the key assumption of this hypothesis, that there is a correlation between migration distance to and arrival date on breeding grounds. This assumption would be fulfilled if departure date was constrained by something, so that birds migrating further could not just leave earlier. I hypothesized that birds were constrained by the need to molt before migrating. Thus, in the next part of the study, expanding on previous work on this species, I looked into whether molt could be limiting the date when birds can leave the wintering grounds. To do this, I used diet manipulation to alter the timing of molt, and then determined the departure date of the manipulated birds. The results supported my prediction that birds given an insufficient diet molted later and departed later than birds given an ample diet, providing support for the hypothesis that molt acts as a constraint on departure. This offers an explanation for why birds cannot leave for the breeding grounds whenever they choose, and thus makes the arrival-time hypothesis for differential migration reasonable. Lastly, to look into the mechanism underlying this relationship, I analyzed the nutritional condition of the experimental birds. I predicted that birds given a poor diet, as a result of being in worse condition, would molt later than their counterparts. To test whether nutritional condition was causing delayed molt I used ptilochronology, the measurement of feather growth rates, to compare birds given ample and insufficient diets. Birds on ample diets grew feathers faster. A comprehensive literature search was done in an attempt to summarize the situations in which ptilochronology seems a useful technique for evaluating condition. The conclusion was that density dependent ecological factors seem to consistently produce changes in feather growth rates, while other types of stressors do not. DIFFERENTIAL MIGRATION, MOLT AND DEPARTURE DATE IN WHITE-THROATED SPARROWS, ZONOTRICHIA ALBICOLLIS