The Mountain White-Crowned Sparrow: Migration and Reproduction at High Altitude
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THE MOUNT~N WHITE-CROWNED SPARROW: MIGRATION AND REPRODUCTION AT HIGH ALTITUDE MARTIN L. MORTON Studies in Avian Biology No. 24 A Publication of the Cooper Ornithological Society THE MOUNTAIN WHITE- CROWNED SPARROW MIGRATION AND REPRODUCTION AT HIGH ALTITUDE Martin L. Morton Biology Department Occidental College Los Angeles, California Studies in Avian Biology No. 24 A PUBLICATION OF THE COOPER ORNITHOLOGICAL SOCIETY Cover drawing of female Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha) attending her nest by Maria Elena Pereyra STUDIES IN AVIAN BIOLOGY Edited by John T Rotenberry Department of Biology University of California Riverside. CA 92521 Artwork by Maria Elena Pereyra Studies in Avian Biology is a series of works too long for The Condor, published at irregular intervals by the Cooper Ornithological Society. Manu- scripts for consideration should be submitted to the editor. Style and format should follow those of previous issues. Price $27.00 including postage and handling. All orders cash in advance; make checks payable to Cooper Ornithological Society. Send orders to Cooper Or- nithological Society, % Western Foundation of Vertebrate Zoology, 439 Calle San Pablo, Camarillo, CA 93010. ISBN: 1-891276-32-8 Library of Congress Control Number: 2002104020 Printed at Allen Press, Inc., Lawrence, Kansas 66044 Issued: June 12, 2002 Copyright 0 by the Cooper Ornithological Society 2002 CONTENTS DEDICATION ..................................................... vi ABSTRACT ....................................................... 1 PREFACE ......................................................... 4 CHAPTER 1: Introduction .......................................... 9 THE ZONOTRICHIA ................................................ 10 FEATURES OF MONTANE ENVIRONMENTS ............................. 1.5 THE STUDY AREA ................................................ 16 CHAPTER 2: Migration Arrival ..................................... 23 ARRIVAL SCHEDULE .............................................. 24 ALTITUDINAL MOVEMENTS ......................................... 28 FORAGING ...................................................... 30 CHAPTER 3: Social System and Behavior ............................ 33 TERRITORY ESTABLISHMENT.. ...................................... 34 PAIRING ........................................................ 34 BETWEEN-YEAR BREEDING DISPERSAL ............................... 35 COPULATIONS .................................................... 38 MATE GUARDING ................................................ 39 MATE SWITCHING ................................................ 39 MATES PER LIFETIME ............................................. 40 AGE OF MATES .................................................. 40 FLOATERS ....................................................... 41 POLYGYNY ...................................................... 42 AGGRESSION.. ................................................... 43 VOCALIZATIONS .................................................. 45 CHAPTER 4: Demography .......................................... 51 LIFE TABLE ..................................................... 52 AGE STRUCTURE OF BREEDING POPULATION .......................... 53 CHAPTER 5: Gonadal Condition .................................... 57 GONADAL CHANGES .............................................. 58 INCUBATION (BROOD) PATCH ....................................... 59 ROLE OF ENVIRONMENTAL CUES IN ANNUAL CYCLES .................. 60 PHOTOPERIOD EFFECTS ............................................ 62 NON-PHOTOPERIOD EFFECTS ........................................ 64 GONADAL HORMONES ............................................. 70 CHAPTER 6: Body Size and Body Condition ......................... 75 WING LENGTH AND SEX .......................................... 76 WING LENGTH AND AGE .......................................... 77 SEASONAL CHANGES IN BODY MASS ................................ 78 DAILY CHANGES IN BODY MASS ................................... 82 CHAPTER 7: Nests and Eggs ....................................... 87 NESTS .......................................................... 88 EGG LAYING .................................................... 94 DESCRIPTIONOF EGGS ............................................ 96 EGG DIMENSIONS ................................................ 96 EGG VOLUME ................................................... 98 WEIGHT Loss OF EGGS DURING INCUBATION ......................... 106 CLUTCH SIZE .................................................... 108 INCUBATION ..................................................... 113 CHAPTER 8: Nestlings and Fledglings ............................... 121 HATCHING ...................................................... 122 BROOD REDUCTION ............................................... 125 HATCHING ASYNCHRONY .......................................... 127 SEX RATIO ...................................................... 131 COWBIRD PARASITISM ............................................. 131 PROVISIONING RATES ............................................. 132 NEST SANITATION ........................... ....... ....... 133 PATTERNS OF PARENTAL CARE ................ ....... ....... 134 GROWTH AND THE~OREGULATION IN NESTLINGS ....... ....... 135 NATAL DISPERSAL.. ......................... ....... ....... 144 CHAPTER 9: Nest Failure .................... ....... ....... 149 PREDATION ................................. ....... ....... 150 DESERTION ................................. ....... ....... 152 STORMS ................................... ....... ....... 153 RENESTING ................................. ....... ....... 157 CHAPTER 10: Reproductive Success ........... ....... ....... 165 ANNUAL REPRODUCTIVE SUCCESS OF NESTS ..... ....... ....... 166 ANNUAL REPRODUCTIVE SUCCESS OF INDIVIDUALS ..... 167 LIFETIME REPRODUCTIVE SUCCESS .................. 168 SNOW CONDITIONS ............................... 170 CHAPTER 11: Late-season Events .................. 179 GONADAL PHOTOREFRACTORINESS ................... 180 MOLT .......................................... 183 TIMING OF SEASONAL BREEDING .................... 192 PREMIGRATORY FA~ENING ........................ 193 MIGRATION DEPARTURE ........................... 197 THE STIMULUS FOR MIGRATION .................... 198 STOPOVER MIGRANTS AND THE MIGRATION SCHEDULE . 199 CHAPTER 12: Concluding Remarks ................ 205 ACKNOWLEDGMENTS .......................... 209 LITERATURE CITED ............................. 210 DEDICATION This monograph is dedicated to Barbara Blanchard DeWolfe with admiration and respect for her pioneering field studies of White-crowned Sparrows and for her career-long support and encour- agement of young scientists. ABSTRACT The reproductive biology of a migratory passerine, the Mountain White-crowned Spar- row (Zonotrichia leucophrys oriantha) was studied for 25 summers in the Sierra Nevada of California at Tioga Pass. Data were obtained on individuals of known age and sex from time of arrival at subalpine breeding meadows to departure for wintering areas in Mexico about four months later. During the summer season many aspects of the reproductive cycle were examined. These included the social system, nesting habits, seasonal and lifetime reproductive success, gonadal development and hormone secretion rate, energy balance as measured by fluctuations in body mass and fat and by doubly-labeled water, molt, and migration departure schedules. Developmental changes in nestlings, along with their sur- vival and dispersal were also investigated. The cardiovascular and respiratory systems of birds pre-adapt them for living at high altitude but achieving reproductive success in mon- tane settings requires adjustments to unusual environmental conditions such as increased solar heating, low nocturnal temperatures, sudden intense storms, and large interannual variations in residual snowpack. Emphasis was placed, therefore, not only on the key features of migration and reproductive biology probably found in all passerine migrants, but also on how these were affected or altered in response to environmental variation. Special attention was paid to underlying physiological mechanisms and this approach, along with the unusual location, helps to distinguish this long-term field study from others. Both sexes tended to return to previously occupied areas although site fidelity was greater in males than in females and mate switching between years occurred in 34.1% of returning pairs. Modal number of mates per lifetime was one and the maximum was six. Pairing usually occurred soon after arrival on the study area but it could be delayed by several weeks in years of deep snowpack. Although females were guarded by their mates, at least one-third of the nestlings were the product of extra-pair fertilizations. Females were aggressive and female-female conflicts sometimes delayed settling by one-year-olds, which were then often shunted to less desirable territories. Polygyny occurred in 3.5% of males, and the number of fledglings produced from their nests increased from 3.1 to 5.5 per season. Fitness in females was unaffected by engagement in polygynous matings. Median time of survival, once one year of age was attained, was 1.9 years for both sexes and survival rate of adults was about 50% per year. This was not different from survival rates found in a sedentary conspecific (Z. 1. nuttalli) so migration itself does not appear to induce extra mortality in White-crowned Sparrows.