Exploring Shifts in Migration Phenology and Breeding Distribution of Declining North American Avian Aerial Insectivores
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EXPLORING SHIFTS IN MIGRATION PHENOLOGY AND BREEDING DISTRIBUTION OF DECLINING NORTH AMERICAN AVIAN AERIAL INSECTIVORES A thesis submitted to the Kent State University Honors College in partial fulfillment of the requirements for University Honors by Nora Honkomp May, 2021 Thesis written by Nora Honkomp Approved by ________________________________________________________________, Advisor ______________________________________,Chair, Department of Biological Sciences Accepted by ___________________________________________________, Dean, Honors College ii TABLE OF CONTENTS LIST OF FIGURES…..……………………………………………….………………….iv LIST OF TABLES………..…………………………………………….………………....v ACKNOWLEDGMENT…………………….…………….……………………………..vi CHAPTERS I. INTRODUCTION……………….………………….…………….………1 II. METHODS……………………….………………………………….…..16 Migration Timing Analysis……………….…………………………..….16 Breeding Distribution Analysis………………………………………..…27 III. RESULTS……………………………………………………………..…30 Migration Timing Analysis………………………………………………30 Breeding Distribution Analysis………………………………………….40 IV. DISCUSSION……………………………………………………………47 LITERATURE CITED…………………………………………………………………..56 APPENDIX........................................................................................................................61 iii LIST OF FIGURES Figure 1. Number of checklists by day of year ………………………......………..……....21 Figure 2. Latitude of sighting by day of year .…………………………………………......22 Figure 3. Start and end dates for spring and fall migration..................................................24 Figure 4. Change in rate of northward movement over time................................................34 Figure 5. Change in rate of southward movement over time................................................35 Figure 6. Day of year of early arrival above the 35th latitude...............................................38 Figure 7. Day of year of late departure above the 35th latitude............................................39 Figure 8. Location of BBS routes consistently surveyed......................................................40 Figure 9. Change in latitude of center of abundance over time............................................43 Figure 10. Change in longitude of center of abundance over time.......................................45 Figure 11. Rate of change in centers of abundance from 1990-2019...................................46 iv LIST OF TABLES Table 1. Population trends of selected avian aerial insectivores …………….……...…..12 Table 2. Four-letter alpha codes and scientific names.......................................................19 Table 3. Comparison of linear regression slopes for spring and fall movement...............33 Table 4. Linear regression results of the latitude of center of abundance by year ...........42 Table 5. Linear regression results of the longitude of center of abundance by year.........44 v ACKNOWLEDGMENTS First and foremost, I would like to thank Dr. Mark Kershner for his vast knowledge, generous guidance, and constant enthusiasm throughout this entire process. I would like to thank Dr. David Singer, Dr. Tim Assal, and Dr. Christie Bahlai for serving as my thesis committee, and for their thoughtful contributions to my work. Next, I would like to thank Ashley Fink and Stephanie Petrycki, as their great efforts in data analysis allowed me to ask a larger research question than I could have handled on my own. A huge thank you to Dr. Shannon Curley who dedicated her time (and code!) to teaching me how to conduct the analysis of breeding distribution and interpret its results. I would also like to show appreciation for all those contributing to the future of avian ecology research including the thousands of volunteers who dedicate their time to completing the BBS routes on an annual basis, the many citizen scientists that record their sightings, and the organizations and individuals that maintain the BBS and eBird databases. Lastly, thank you to my family, roommates, and friends for their constant support and encouragement throughout this process. vi 1 Introduction Recent studies have shown major declines in North American bird populations. In fact, a study by Rosenberg et al. (2019) described the cumulative loss of 3 billion birds over the last five decades, representing a 29% decline in bird abundance since 1970. Further, it is particularly of concern that 2.5 billion individuals of this loss are migratory species (Rosenberg et al. 2019). There are many, multi-faceted reasons as to why these declines are taking place (Loss et al. 2015), including effects of climate change on migration and subsequent breeding. Given that migratory species make up such a large proportion of missing birds, it is important to look at migration and its challenges in order to understand the major drivers of these losses. Migration Many species use large-scale migrations as a life history strategy to increase fitness. Birds are especially notorious for their annual migrations between breeding and wintering grounds, given their conspicuous and charismatic natures. Typically, migratory species will travel to a specific, set breeding location from a specific wintering range (where they will spend the non-breeding season). Some species, known as ‘partial migrants', only travel a short distance from wintering to breeding grounds, such as up the side of a mountain or up/down a few degrees in latitude at the change in seasons, and often remain in parts of their breeding range year-round. 2 In contrast, long-distance travelers, known as “neotropical migrants”, spend the breeding season (spring and summer) in North America and the non-breeding season (winter) in Central and South America, travelling thousands of miles twice a year between breeding and wintering grounds. During their pre-breeding migration (“spring migration”) and post-breeding migration (“fall migration”), they will travel through areas of the continent where they do not typically breed. These areas are considered part of a species’ migratory route and will see neotropical migrants for a short period of time each year during spring and fall (Cornell 2019). The benefits of these immense, long-distance journeys must be worthwhile considering the amount of time and energy they require, as well as the many possible risks they face during migration (Loss et al. 2015, Cornell 2019). The necessity of migration is likely linked with the pressures for survival and reproduction. For example, breeding grounds need to provide enough food for adult survival, and the provisioning necessary to raise young. Further, there are additive pressures associated with finding ideal climatic conditions, habitat type, space or territories, mates, etc. that affect their choice in breeding grounds. For neotropical migrants, winters in temperate regions of North America are too cold and do not offer enough food to sustain their populations, as most plants no longer produce seeds or fruits and most invertebrates are dormant. For this reason, they spend these cold months in tropical and subtropical regions where temperature and precipitation conditions are ideal and food is abundant among the many plant and invertebrate species that are active throughout the year. 3 With plentiful resources and ideal temperatures, the tropical areas may seem like a prime place to stay year-round and even raise young there. However, this is not the case as intense competition exists for the resources and space in these areas from the multitude of tropical species present. Long-distance migrations are thought to have evolved from the advantage of greater food availability in temperate regions which provides the ability for these species to raise more offspring in these areas (Cornell 2019). Additionally, wet and dry seasons change environmental conditions in tropical areas the same way the four seasons shift temperature and precipitation conditions in the temperate regions, leaving periods of unsuitable abiotic conditions for migrant species. To balance this temporal trade-off in a way that maximizes access to resources and survivable environmental conditions, neotropical migrants are adapted to travel between the wintering and breeding environments despite the massive energy cost of prolonged flight and many risks (Cornell 2019). Given these pressures, migratory birds must determine when to depart so that they arrive on the breeding or wintering grounds at the appropriate time. It is impossible to predict what environmental conditions may be thousands of miles away, so the birds must rely on annually consistent cues to predict when their breeding and wintering grounds may be offering favorable conditions. Though the mechanisms used to determine when to begin migration are not entirely understood for every species, there are multiple hypotheses on which factors may contribute. It is possible that the birds detect changes in temperature, precipitation, and weather patterns and use these abiotic factors as potential cues (Cornell 2019). In conjunction, the birds may be attuned to other species in the area 4 for determining when the proper time to depart for the northern latitudes may be, particular those related to their food supply (Studds and Marra 2011). Highly variable weather cues are most commonly used by shorter-distance migrants and allow flexibility in timing between years (Hagan et al. 1991). Long-distance migrants, however, have much more consistent departure dates, and are unlikely to use these variable cues (Schwemmer et al. 2021). Birds may also use changes in photoperiod