W&M ScholarWorks Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects 2001 Differential Migration, Molt and Departure Date in White-Throated Sparrows, Zonotrichia albicollis Kendell Jenkins College of William & Mary - Arts & Sciences Follow this and additional works at: https://scholarworks.wm.edu/etd Part of the Zoology Commons Recommended Citation Jenkins, Kendell, "Differential Migration, Molt and Departure Date in White-Throated Sparrows, Zonotrichia albicollis" (2001). Dissertations, Theses, and Masters Projects. Paper 1539626305. https://dx.doi.org/doi:10.21220/s2-f7cp-4836 This Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. DIFFERENTIAL MIGRATION, MOLT AND DEPARTURE DATE IN WHITE-THROATED SPARROWS, ZONOTRICHIA ALBICOLLIS A Thesis Presented to The Faculty of the Department of Biology The College of William and Mary in Virginia In Partial Fulfillment Of the Requirements for the Degree of Master of Arts by Kendell Jenkins May, 2001 ProQuest Number: 10630084 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10630084 Published by ProQuest LLC (2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 APPROVAL SHEET This thesis is submitted in partial fulfillment of the requirements for the degree of Master of Arts Kendell D. Jenkins Approved, May 2001 Daniel A. Cristol, Ph.D. Paul D. Heideman, Ph.D. Stewart A. Ware, Ph.D TABLE OF CONTENTS Page ACKNOWLEDGEMENTS iv LIST OF TABLES v LIST OF FIGURES vi ABSTRACT vii INTRODUCTION 2 CHAPTER 1. DIFFERENTIAL MIGRATION 15 CHAPTER II. DIET, MOLT, AND DEPARTURE 28 CHAPTER III. NUTRITION AND FEATHER GROWTH 41 SUMMARY AND CONCLUSIONS 52 LITERATURE CITED 55 iii ACKNOWLEDGEMENTS i I am extremely grateful for the guidance and support given by Dan Cristol throughout this study. In addition, I am very appreciative of the time put forth by Stewart Ware and Paul Heideman, and their comments helping to improve this thesis. I would also like to thank the following William & Mary students who were involved in capturing and/or observing birds: Jessica Armstrong, Dave Cerasale, Chris Farabaugh, Dee Fritz, Kris Gleason, Katie Martin, Andrew McCormick, Katie Murphy, Emily Snell-Rood, Mary Kate Savitske, Zakiya Thomas, and Magill Weber. I also acknowledge the following museums and each of their staffs: Academy of Natural Sciences, American Museum of Natural History, Cornell University Museum of Vertebrates, Louisiana State University Museum of Natural Sciences, Museum of Comparative Zoology at Harvard, Museum of Vertebrate Zoology at Berkeley, National Museum of Natural History, Natural History Museum of Los Angeles County, North Carolina Museum of Natural Sciences, Cincinnati Museum of Natural History and Science, University of Michigan Museum of Zoology and Virginia Tech Museum of Natural History. In addition, I am grateful to Chris Witt, Jason Mobley, Chris Farabaugh and Kevin Croll for measuring birds at museums. I am thankful for all of the hours that Chris Farabaugh put in carrying out ptilochronology. Lastly, I thank Kathy Klimkiewicz and the USFWS Bird Banding Lab, as well as Walter Piper and Charles Zeigenfus for sharing sex-ratio data. Also, I would like to express my appreciation to Charles Ziegenfus for introducing me to ornithology. Lastly, I am very appreciative of all the support and patience from my family and friends throughout this project. Funding for this research was provided by grants from the Virginia Academy of Sciences, the College of William and Mary, and the Wilson Ornithological Society. LIST OF TABLES Table Page 1. Bird Banding Lab data for all latitudes with sexed birds in each flyway ...................21b 2. Museum data for all latitudes with sexed birds for Atlantic and central flyways....23a 3. Expected and observed proportions of females by latitude from museum skin data .....................................................................................................23b 4. Expected and observed proportions of females found at field sites.......................... 23c 5. Molt and departure statistics (Mean + SD) of experimental and free-living control White-throated Sparrows........................................................37a 6. Comparisons of fat, mass, indices of mass corrected by wing and tarsus and feather growth for birds receiving an ample (ADV) or insufficient (DEL) diet ............ 46a 7. Nutritional variables positively related (+), negatively related (-), or unrelated (o) to feather growth rate in studies using ptilochronology.......... 48a 8. Environmental variables positively related (+), negatively related (-), or unrelated (o) to feather growth rate in studies using ptilo chronology.......... 48b LIST OF FIGURES Figure Page 1. Relationship between latitude and percent of females in the Atlantic and central flyways based on Bird Banding Lab data ................................................. 21a 2. Mean (+ SE unless SE < 1.0) molt scores for each treatment group.......................... 37b 3. Original and induced growth bar widths (+ SE) by treatment group ......................... 46b vi ABSTRACT 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 White- throated 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 INTRODUCTION Migration In migratory species, long-distance travel between wintering and breeding grounds evolved because it allowed migrant individuals