Response of roosting aerial insectivores to ongoing climate change quantified by weather surveillance radar

Abstract

Aerial biotas encompass species that feed on airborne insects while in flight, including various bird species (e.g., swallows, martins, nightjars, and flycatchers), bats, and insects (e.g., dragonflies). This community inhabits aerial habitats and occupies a critical interface between terrestrial and aquatic ecosystems. Because these ecosystems provide essential food resources and habitats, aerial insectivores' responses to climate change are closely tied to shifts at these ecological boundaries. As climate change intensifies, these habitats are increasingly unpredictable due to long-term climate shifts and more frequent extreme weather events, like cold snaps. These environmental changes may have driven biological changes at varying levels across aerial trophic food webs, potentially leading to trophic mismatches—where the timing of consumer demand no longer aligns with the availability of their food resources. As a result, aerial insectivores are particularly vulnerable to these shifts, and many are experiencing acute population declines. As key predators in the aerial trophic system, aerial insectivores help maintain ecosystem balance and health. They also provide ecosystem services, such as pest population control and the reduction of insect-borne diseases. Despite their ecological and agricultural importance, knowledge gaps remain regarding how aerial insectivores respond to environmental changes at a macrosystem scale—particularly regarding shifts in phenology and population dynamics. Addressing this knowledge gap is crucial for understanding and mitigating the cascading effects of these changes on ecosystems. Therefore, there is a need to monitor changes in aerial insectivore populations and phenology. In North America, some aerial insectivore species form large roosting aggregations—sometimes numbering in tens of thousands—during certain life cycle stages. These roosts appear on Next Generation Weather Radars (NEXRAD) as expanding "angel rings," providing a unique opportunity to quantify their occurrence across broad spatial and temporal scales. To leverage this, I integrated decades of radar data with other remote sensing datasets and machine learning tools to quantify phenological and population changes and to investigate their drivers in two taxonomic groups of roosting aerial insectivores: swallows and bats. My dissertation centers around the key question: How has the phenology and population of these aerial insectivores changed in response to changing climate? To address this, I first focused on roosting swallows in the Great Lakes region, primarily Tree Swallows and Purple Martins. Chapter 1 asks How swallow roosting phenology changed over the past two decades? Chapter 2 asks Which aspects of climate change, during which periods, at what locations, impact swallow roosting phenology at the Great Lakes? In Chapters 3 and 4, I shifted my focus to Mexican free-tailed bats in south-central Texas. Chapter 3 asks Has phenology and population of Mexican free-tailed bats changed in this region? Chapter 4 asks Can I predict the intensity of nightly emergence events of Mexican free-tailed bats? The goal of my work is to provide ecological evidence of these changes and uncover the underlying mechanisms, offering insights that can inform conservation policies and actions.