Radar revelations: insect availability influences parental provisioning in breeding tree swallows Tachycineta bicolor

Abstract

Airspace habitat is essential foraging space for tree swallows Tachycineta bicolor, which rely on flying insects as their main source of food. Insect availability can change quickly from hour-to-hour or day-to-day, however, it is unclear whether insectivores primarily respond to changing atmospheric dynamics, resource dynamics, or a combination. Rapidly changing conditions are common in high-elevation areas – an understudied portion of the tree swallow's breeding range. To explore the relationship between food availability and high-elevation weather conditions as related to female provisioning, we deployed a mobile radar unit to collect insect abundance data during the 2022 and 2023 summer breeding seasons at a high-elevation site in Colorado's Rocky Mountains. We monitored 41 active nest boxes using radio-frequency identification (RFID) technology to track female provisioning behavior. We deployed three models to assess 1) how strongly swallow provisioning rates correlated with insect traffic rates, 2) how well swallow provisioning rates were explained by insect traffic rates and weather conditions, and 3) how insect traffic rates were related to weather conditions. Although there remains substantial unexplained variation in tree swallow provisioning rates, we found a significant positive relationship with insect traffic rate, a negative relationship with precipitation, and curvilinear relationships with temperature and wind speed. Weather variables and time of day explained nearly 80% of the variation in insect traffic rate, and the strength of these relationships suggests weather conditions serve as a good proxy of airborne insect activity. This research presents a link between our vast airspace habitat and animal ecology, advancing our understanding of how flying organisms respond to rapidly changing conditions in aerial environments and how multiple factors contribute to variation in provisioning rates in an aerial insectivore.