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

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TABLE OF CONTENTS
LIST OF FIGURES…..……………………………………………….………………….iv LIST OF TABLES………..…………………………………………….………………....v ACKNOWLEDGMENT…………………….…………….……………………………..vi CHAPTERS

• I.
• INTRODUCTION……………….………………….…………….………1

METHODS……………………….………………………………….…..16 Migration Timing Analysis……………….…………………………..….16 Breeding Distribution Analysis………………………………………..…27 RESULTS……………………………………………………………..…30 Migration Timing Analysis………………………………………………30 Breeding Distribution Analysis………………………………………….40 DISCUSSION……………………………………………………………47
II. III. IV.
LITERATURE CITED…………………………………………………………………..56 APPENDIX........................................................................................................................61

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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 35^th latitude...............................................38 Figure 7. Day of year of late departure above the 35^th 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

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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

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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.

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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.
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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.
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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
4for 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 to predict when they should begin migrating as it will provide annually consistent cues for prime departure date. Interestingly, while photoperiod may be a valuable source of phenological information in temperate regions, changes in day length and position of the sun are much more subtle near the equator where wintering neotropical migrants would be when determining the right time to leave for the breeding grounds. Because of this, it is thought that endogenous cues and genetic triggers are likely to determine departure date for these species (Hagan et al. 1991).
Two final aspects of migration to understand are the route that a given species uses when migrating and at what rate they travel. Across North America, four flyways exist that channel most species from Mexico towards Canada and back; these are the Pacific, Central, Mississippi, and Atlantic flyways. As the migrants leave Central America, some populations take the long route in which they fly over land, moving their way up through Mexico, while others take a more direct route, crossing over the Gulf of Mexico. It is common for the routes a population uses to travel north in the spring to differ from the route they use to travel south in the fall (La Sorte et al. 2013). In terms of rate of migration, several factors may be at play. Bigger species tend to migrate faster (La
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Sorte et al. 2013). Some are able to fly very far distances and then stop for long periods of time to rest and refuel before making their next long flight towards their destination. Others travel in “hops” where they fly short distances, stop for a small amount of time, and then fly another short distance (Klaassen 1996). While not understood as well, species may travel at a different rate during fall in comparison to spring since they may use different routes, face different pressures, and do not have the pressure of needing to get to the breeding grounds.

Challenges of Migration

Along their migration routes, birds face many natural obstacles for survival. The risk of predation is heightened during migration as birds travel for long periods of time through open air and encounter many aerial predators (Cornell 2019). Along with predation, some migrants experience strong competition. In an effort to arrive at the breeding grounds first to obtain the best territories and nesting locations, some species participate in an intraspecific race to more northern latitudes. This however, can result in arrival during a time that does not yet offer favorable weather conditions. A sudden cold snap in the spring can lead to lack of food and a day spent huddled together instead of foraging. These environmental factors faced upon arrival compound difficulties experienced during their stressful journey. During migration, birds deal with many physiological constraints involving energy storage and water retention, as they need to maintain a specific weight for optimal flight, requiring set intervals of refueling at stop-
6over sites and optimal food availability that offers the right balance of fats and proteins (Klaassen 1996).
In addition to the natural challenges associated with migration, humans have added many obstacles to this already incredible feat. The leading anthropogenic mortality factor for birds is predation by domestic cats, which kill billions of birds each year (Loss et al. 2015). Following this, collisions with buildings and automobiles each kill hundreds of millions of birds on an annual basis (Loss et al. 2015). Collisions and electrocution at powerlines, collisions with communication towers and wind turbines, and poisoning from agricultural chemicals all contribute millions of bird deaths each year as well (Loss et al. 2015). These factors are direct and measurable, but there are many indirect mortality factors caused by humans that are harder to quantify. The loss of habitat for nesting and foraging offer challenges to survival and reproduction on the breeding grounds, on the wintering grounds, and on the stopover sites in between. Land transformation for urbanization and agricultural use are generally the driving factors behind habitat loss. The introduction of non-native species can increase predation risk and disease transmission. Additionally, the altered plant communities created by introduction of exotic species can result in a decrease in food availability (Narango et al. 2018). Each of these factors contributes to large declines in neotropical migrant populations as death rates are higher than birth rates, meaning more birds die without being able to replace themselves.
Ultimately, climate change is one major human impact occurring at a global scale that threatens entire migratory guilds by reducing the environmental and biotic predictability that these animals so greatly rely on for migration and reproduction. The
7first impact of climate change relates to the timing of food availability. Changing abiotic factors can lead to new timing for biological processes like spring green-up, insect emergence, and vertebrate breeding seasons (Scranton and Amarasekare 2017). However, various trophic and taxonomic groups have differing levels of sensitivity to climate change (Voigt et al. 2003, Thackeray 2016). This diversity leads to different rates of advancement or delays in timing between biological interactions within food webs and ecosystems. The disparity between the phenology (timing) of organisms that rely on each other is referred to as “phenological mismatch” or “trophic asynchrony” (Renner and Zohner 2018). For example, birds are known to track vegetation greenness in the spring and fall (La Sorte and Graham 2020), which allows them to synchronize breeding and rearing of young with peak food abundance. When the food source advances its emergence due to warmer springs, the birds may not be able to adapt their timing to meet this change and breeding/reproduction can be negatively impacted (Doiron 2015).
Climate change also poses threats for habitat availability, which impacts bird distribution. Currently, two thirds of North American bird species have moderate or high vulnerability to range loss, range gain, and ineffective dispersal abilities under a 3^oC temperature increase (Bateman et al. 2020a). A follow up study found that under unmitigated climate change, 88% of the contiguous United States will be affected by various climate change-related threats (such as sea-level rise, drought, and increased storm severity) and 97% of North American bird species will be affected by at least two of those threats (Bateman et al. 2020b). Further, neotropical migrants are expected to experience decreased rainfall on their wintering grounds and increased temperatures on
8their breeding grounds as well as along migration routes (Studds and Marra 2011, La Sorte et al. 2017). These factors pose threats to the areas in which birds have historically chosen to breed and winter, resulting in large portions of species’ ranges becoming less habitable/uninhabitable for them.
Climate change will impact not only the destinations, but also the journey itself.
Changing wind patterns during spring and fall migration can affect birds’ rate of movement and energy expenditure. Based on potential changes in North American wind patterns, spring migration may become more efficient with respect to energy use while fall migrations may become less efficient (La Sorte et al. 2019a). Further, changing wind patterns may result in faster rates of migration in spring and slower rates of migration in fall (La Sorte et al. 2019a). Finally, birds will likely face conflicting pressures relative to migration timing over the next century as novel climates emerge on differing timelines on wintering grounds relative to breeding grounds (La Sorte et al. 2019b). Ultimately, the ability of Neotropical migrants to travel at the same rate, to the same places, at the same time of year may no longer be an option later this century.

Bird Response to Novel Stressors

There is hope, however, that birds may be able to set a new schedule to keep up with availability of preferred food resources and abiotic conditions. In fact, trophic asynchrony is expected to last for an evolutionarily short amount of time due to strong pressures for adaptation (Renner and Zohner 2018). In fact, some migratory birds are already changing migration timing in response to changing temperatures (Zaifman et al.
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2017). When assessing the potential for phenological mismatch, 48 North American bird species have altered their spring arrival dates in the same direction as the advancing green-up date (Mayor et al. 2017). Of those species, 39 were able to keep pace with the advancement or delay of spring green-up (Mayor et al. 2017). Weather radar data has shown that the timing of both spring and fall migration has advanced across the contiguous United States during the past 24 years and these changes can be linked to warmer seasons (Horton et al. 2020). However, while the timing of peak bird passage over the Gulf of Mexico did not change from 1995 through 2015, there was a 3-day advancement of the earliest migrants passing over, and this change was linked to species with larger body size and shorter migration distance (Horton et al. 2019). Shifts in migration timing are becoming well documented, particularly relative to spring arrival on the breeding grounds.
Fall migration is less understood overall as fewer studies have focused on this. It is harder to detect and identify migrating individuals once breeding season has ended as they reduce their singing frequency and may no longer sport bright, distinctive breeding plumage. Further, effects of climate change on the timing of fall migration are less understood and are considered to be more dependent on species-specific life history traits that result in less clear trends (Jenni and Kéry 2003). In fact, species may delay or advance their fall migration based on conditions they will face along their migration route and whether they have the ability to attempt a second brood (Jenni and Kéry 2003). Despite the difficulties of interpreting differing trends and species-specific variation, fall
10 migration is important to study as it may complicate birds’ abilities to adapt their life cycle so as to promote a successful breeding season in the following year.
In response to newly unsuitable conditions in portions of migrants’ wintering and breeding grounds, there may also be some flexibility relative to distribution shifts. Many terrestrial species are changing their ranges through latitudinal shifts, with the magnitude and rate of these changes being species-dependent (Chen et al. 2011, Hovick et al. 2016). Ability to move provides these organisms with the opportunity to take advantage of previously uninhabitable areas by simply shifting the range northward. However, a caveat to this type of range shift includes consideration of ‘range compression’. While resident and partial migrant species in North America have been found to shift their range northward by expanding the northern leading edge and holding the southern edge constant, neotropical migrants may shift their range by holding their northern border constant and shifting their lower border northwards (Rushing et al. 2020). This introduces concern for habitat availability and reproductive success on the breeding grounds of longdistance migrants.

This Study

Based upon my personal interests and the topics covered above, I chose to investigate species-specific shifts in migration timing and breeding distribution for a range of bird species for my Honors thesis. In this study, I used large datasets collected through citizen science efforts and annual surveys. My species of interest are all members of a specific avian foraging guild, the aerial insectivores, that primarily breed in the
11 eastern United States. By taking a guild-based approach, I can assess variation among individual species and make comparisons based on a shared feeding strategy.
Nineteen species are included in this analysis. These include three nightjar species
(Common Nighthawk, Eastern Whip-poor-will, and Chuck-will’s-widow), one swift species (Chimney Swift), five swallow species (Barn Swallow, Bank Swallow, Cliff Swallow, Northern Rough-winged Swallow, and Purple Martin), and ten flycatcher species (Great Crested Flycatcher, Eastern Kingbird, Acadian Flycatcher, Alder Flycatcher, Willow Flycatcher, Least Flycatcher, Olive-sided Flycatcher, Yellow-bellied Flycatcher, Eastern Phoebe, and Eastern Wood-Pewee). These species share the common trait of foraging for insects while in flight. Unfortunately, they also share the reality of greater declines in the past few decades when compared to other North American bird species. Aerial insectivore populations have declined 32% from 1970 population levels (Rosenberg et al. 2019). In fact, based upon the North American Breeding Bird Survey 2020 Analysis of Trends (Sauer et al. 2020), 15 of the 19 aerial insectivores included in this study have seen significant population declines from 1966 through 2019 (Table 1).