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

Submitted by Michi on 6 May 2025.

Photo above: The BirdScan MR-1 Radar deployed in the field. Photo credit: Kyle Horton.

Text by Victoria Simons

Tree swallows (Tachycineta bicolor) are considered a model organism in the field of ecology, and they are commonly viewed as an indicator species for the health of semi-aquatic habitats. While tree swallows have been the focal species in hundreds of studies, little is known about their high elevation populations. High elevations undergo rapid shifts in daily weather, requiring organisms to adapt quickly to changing conditions. This is especially true for aerial insectivores that rely on dynamic airspace habitats. Our study aimed to address this gap by researching a breeding population of swallows at Colorado State University’s Mountain Campus. We wanted to know if atmospheric conditions such as temperature, precipitation, wind speed, and aerial insect prey availability have an influence on female tree swallow breeding success, and we chose to view this question through the lens of provisioning behavior – how parents provide food for their growing young.

To answer this question, we combined traditional field methods of next box monitoring with radio-frequency identification (RFID) technology for automated detection of provisioning visits that each female made to her nest box. To do this, we captured females by trapping them inside their nest boxes while they incubated their eggs and quickly fit them with a passive integrated transmitter (PIT) tag that would be detected by an antenna at the nest box entrance. We also deployed a BirdScan MR-1 mobile radar unit to detect flying insect activity overhead. We then compared these data to hourly measures of temperature, precipitation, and wind speed collection by weather instrumentation on-site.

                                                        Victoria fitting a PIT tag onto a captured tree swallow. Photo credit: Kyle Horton.

We found that aerial insect availability, precipitation, temperature, and wind speed all play a role in shaping tree swallow provisioning behavior. Additionally, we found that aerial insect activity is closely related to weather conditions. During cold, wet, or windy conditions, fewer insects are flying in the airspace, which means prey availability is reduced for foraging tree swallows. This translates to challenges for nestlings, who require a high energy input for successful growth and development.

                                                                     Colorado State University’s Mountain Campus. Photo credit: Victoria Simons.

This study furthers our understanding of airspace habitats by linking atmospheric conditions, prey availability, and provisioning behavior. As broad-scale weather patterns begin to shift as a result of climate change, our findings indicate that tree swallows and other aerial insectivores may face increased challenges. High elevations are at the forefront of climate change impacts, and populations in these areas may struggle to persist under increased risks to nestling survival.

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