STOPOVER ECOLOGY OF THE BLACKPOLL WARBLER, Dendroica striata, DURING SPRING MIGRATION ON THE COASTAL PLAIN OF VIRGINIA A Thesis Presented to The Faculty of the Department of Biology The College of William & Mary in Virginia In Partial Fulfillment Of the Requirements for the Degree of Master of Arts by Magill Weber 2000 APPROVAL SHEET This thesis is submitted in partial fulfillment of The requirements for the degree of Master of Arts ..,4 z fA-- Magill Weber Approved, August 2000 0~ Daniel A. Cristol, Ph.D. ~,~ Stewart A. Ware, Ph.D. Xi 12 ZM j{~. Watts, Ph.D. Committee Chairman! Advisor 11 • TABLE OF CONTENTS ACKNOWLEDGEMENTS v LIST OF FIGURES vi LIST OF TABLES vii ABSTRACT viii GENERAL INTRODUCTION 1 General Introduction 2 Literature Cited 6 CHAPTER 1. DISTRIBUTION AND TIMING OF MIGRATION 9 Introduction 10 Methods 11 Results 13 Discussion : 16 Literature Cited 18 Figure 1 15 CHAPTER 2. FORAGING ECOLOGY ANDENERGETICS 20 Introduction 21 Methods 22 Results '" 25 Discussion 34 Literature Cited 37 Table 1 29 Table 2 31 iii Table 3 33 Figure 1 27 Figure 2 30 GENERAL DISCUSSION 40 General Discussion 41 Literature Cited 46 VITA 48 IV ACKNOWLEDGEMENTS I would like to thank Dr. Bryan Watts for providing assistance, direction, and insight throughout all phases of the research process. Ialso would like to thank my committee members, Dr. Dan Cristol, and Dr. Stewart Ware for their help, advice, and support. Iwould also like to thank other students in the Biology Department, and especially Dave Cerasale, Kendell Jenkins, Renae Held, and Amanda Kaye for their assistance in the field. Lastly, Iwould like to thank my parents, who encouraged this endeavor. And especially Iwould like to thank Garrett McKnight, who tolerated two years of pre-dawn wakeups, and sustained bodily injury in the pursuit of Blackpoll Warblers. This project would not have been successful without property access provided by the many landowners in the neighborhoods of Forest HiIls in Richmond, Queens Lake and Kingswood in Williamsburg, Bellaire in Norfolk, and Throughgood in Virginia Beach. Many thanks also go to The Maymont Foundation and the City of Richmond Parks Department for providing land access. Financial support for this project came from The Center for Conservation Biology, The Eastern Bird Banding Association, The Virginia Society for Ornithology, The College of William & Mary's minor research grant fund, and in the form a teaching assistantship from the College's Department of Biology. This project represents the culmination of a life long fascination with birds and birding. I remember seeing my first Blackpoll in my parent's backyard. It was the spring of 1984. Decades later Icontinue to be in awe of the amazing journey of North America's longest distance migrant. v LIST OF FIGURES Figure Page 1.1. Migration phenology by geographic area 15 2.1. Blackpoll Warbler time budget.. .27 2.2. Prey size distribution taken by foraging Blackpoll Warblers '" 30 VI LIST OF TABLES Table Page 2.1. Frequency and time spent using specific foraging tactics by Blackpoll Warblers 29 2.2. Foraging tactics and taxa of prey items used by Blackpoll Warblers 31 2.3. Foraging rates as a function of foraging tactics 33 VII ABSTRACT High quality stopover sites along the migratory path are critical to migrant songbird populations because arrival timing on the breeding grounds impacts breeding success. Both sexes gain a reproductive advantage by arriving on the breeding territory with extra fat reserves (Sandberg and Moore 1996) obtained during stopovers. During the spring, Blackpoll Warblers, Dendroica striata, stopover in large numbers on Virginia's coastal plain (Burleigh 1934). In this study, I set out to examine the energetic value of the region to migrant Blackpoll Warblers by examining the geographic patterns of migration, generalizing foraging ecology of Blackpoll Warblers, and determining whether this region represents a source of energy for migrants. It appears Blackpoll Warblers are responding to available energy resources, and that this region represents a source of energy for Blackpoll Warblers during the spring migration. Vlll 2 INTRODUCTION Bird migration is closely tied to seasonal food resources (Alerstam 1990). The annual surge in insect populations across the northern temperate zone provides an enormous food resource for breeding songbirds. Numerous species have evolved to take advantage of this resource, by migrating from the tropics to breed in nearctic regions (Alerstam 1990). There are tradeoffs associated with this migration. Instead of remaining in a harsh climate throughout the year, Neotropical migrants breed in areas with abundant arthropod resources, and winter in areas of mild climate and relatively constant food source. However, migrants encounter unpredictable wind, weather, and food conditions along the migratory pathway. Blackpoll Warblers breed near the ground in stunted spruce forests and alder thickets across the northern tier of Alaska and Canada, and at high elevations in the Canada's Maritime Provinces and New England. In the winter, the population resides along the edges of lowland tropical fores~s in Columbia, Peru, Bolivia, and Brazil (Curson et al. 1994). Annual migration for Blackpoll Warblers can exceed 16,000 km roundtrip (Nesbit et al., 1995). The route of fall migration has been the subject of much debate in the last 40 years. Blackpoll Warblers have been found to stage in large numbers in New England and the Canadian Maritime Provinces, storing enormous fat loads for several weeks. The birds then depart with favorable winds out over the Atlantic (Nesbit et al 1963, McNair and Post 1993, Nesbit et al. 1995). This route is currently accepted as the standard migratory path (Hunt and Eliason 1999), but there has been debate as to whether it is 3 followed by the bulk of the population (Murray 1989). Blackpolls have been shown to be physiologically capable of completing this migratory route (Nesbit et al. 1995), and birds have been found along the proposed trajectory on the island of Bermuda (Nesbit 1963). Additionally, numerous offshore sightings (Cherry et al. 1985) substantiate the route. The spring migration route has not been well studied. High quality stopover sites along the migratory path are critical to migrant songbird populations because arrival timing on the breeding grounds impacts breeding success. For males, arrival timing is directly linked to productivity (Klomp 1970), and it is important that they return to the breeding area during a specific window of time. The most productive individuals are the ones that have good food availability at the time of nesting, and have nests hatching at the time of greatest food abundance (Perrins 1970).In order to achieve hatching within this window of time, it is important that males arrive early enough to establish and defend territories, and attract mates (Francis and Cooke 1986, Gauthreaux 1982). Females are not under extreme competition for mates and territories, but depend equally on high quality stopover sites, because they face high energetic costs associated with egg production. Early female arrival is directly correlated with breeding success as well (Murphy 1986). Both sexes gain a reproductive advantage by arriving on the breeding territory with extra fat reserves (Sandberg and Moore 1996) obtained during stopovers. During spring migration, stopover sites are critical to Blackpoll Warblers. An individual may stop in a location in an attempt to fulfill energetic requirements (Moore et al. 1995), though exact mechanisms for fine scale site selection are not known. Larval arthropods are thought to provide the best source of energy for migrants because of their 4 high water content and caloric value (Graber and Graber 1983). It is unlikely that individuals that do not meet metabolic requirements for migration will survive the trip. The spring stopover ecology of migrant warblers has been well studied along the Gulf of Mexico, where migrants make landfall and immediately begin utilizing arthropod resources to replenish fat lost during the crossing. Blackpoll Warblers, following spring trans-Gulf migration, had an average stopover length of 2.4-3.3 days (Kuenzi and Moore 1991). Migrants that stopped over for longer, gained more mass (Moore and Kerlinger 1987). Previous work with Blackpoll Warblers during migratory stopover indicates that the species tolerates a wide range of habitats. In the Dominican Republic, Blackpoll Warblers were found in low mangrove thickets, foraging on larval arthropods (Latta and Brown 1999). In North Carolina, they were found in a mix of thickets, hardwood forests, and pine stands (Parnell 1969), and in Maryland they were found along mature hardwood forest edges, and in park-like settings with little or no understory (Morse 1979). During the spring, Blackpoll Warblers stopover in large numbers on Virginia's coastal plain (Burleigh 1934). Along the coastal plain of Virginia, their arrival tends to coincide with annual spring leaf onset in deciduous trees (per ob.). Leaf onset is determined by a combination of factors including latitude, average spring temperatures, rainfall, and proximity to the coast. Trees on the Coastal Plain of Virginia will leaf before those in the Piedmont and in points further north. Consequently, it is possible that there may be a large supply of arthropods on the coastal plain during the time that Blackpoll Warblers are passing through the region. 5 Additionally, an overwater spring migratory route has been suggested for Blackpoll