Radar revelations: Insect availability influences parental provisioning in breeding Tree Swallows (Tachycineta bicolor)

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 the 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, negative relationship with precipitation, and curvilinear
relationships with temperature and wind speed. Weather variables and time
of day explained nearly 80% of variation in insect traffic rate, and the
strength of these relationships suggest 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.

空域生境是树燕（Tachycineta bicolor）至关重要的觅食空间，这类鸟类以飞行昆虫作为主要食物来源。昆虫资源的可获得性可能在小时级乃至日级尺度上快速波动，但目前尚不明确食虫动物主要对何种变化做出响应：是大气动态变化、资源动态变化，抑或是二者的共同作用。快速变化的环境条件在高海拔区域极为常见，而该区域恰好是树燕繁殖分布范围内研究不足的部分。为探究食物可获得性、高海拔气象条件与雌性亲鸟育雏行为之间的关联，研究团队于2022年和2023年夏季繁殖季，在科罗拉多州落基山脉的一处高海拔研究站点部署了移动雷达设备，以采集昆虫丰度数据。研究团队采用射频识别（Radio-Frequency Identification, RFID）技术，对41个正在使用的巢箱进行监测，以追踪雌性亲鸟的育雏行为。本研究构建了三类模型，分别用于：1）量化树燕育雏频次与昆虫飞行通量之间的相关强度；2）评估昆虫飞行通量与气象条件对树燕育雏频次的解释能力；3）探究昆虫飞行通量与气象条件之间的关联。尽管树燕的育雏频次仍存在大量未被解释的变异，但研究结果显示：其育雏频次与昆虫飞行通量呈显著正相关，与降水量呈显著负相关，而与温度和风速则呈曲线性关系。气象变量与时段可解释近80%的昆虫飞行通量变异，且这些关联的强度表明，气象条件可作为空中昆虫活动水平的良好替代指标。本研究搭建了广袤空域生境与动物生态学之间的关联桥梁，加深了学界对飞行生物如何响应空中环境快速变化、以及多重因素如何影响食虫飞行动物育雏频次变异的认知。