Nestling Diet, Quality, and Post-fledging Survival: Comparing Two Species of Aerial Insectivores in a Gradient of Agricultural Intensity in Saskatchewan, Canada

In the last decades, several species of avian aerial insectivores have experienced steep declines in different regions of North America, and no single driver has been identified. Given the common reliance of aerial insectivores on flying insects, changes in spatiotemporal availability of prey abundance and quality caused by agricultural intensification have been implicated in their decline. The quality of nestling nutrition mediated by parental selection of insect prey types (aquatic versus terrestrial) can be an important determinant of offspring health and development which, in turn, can have lasting effects on future life-stages, including the post-fledging transitional period to independence. In 2020 and 2021, using a comparative approach between two sympatric species, Barn Swallows (Hirundo rustica) and Tree Swallows (Tachycineta bicolor), following a scale-of-effect approach, I investigated whether the intensity of agriculture was associated with parental selection of prey resources, nestling body condition and growth. Stable isotope data revealed Barn Swallows opportunistically used aquatic aerial prey resources to provision nestlings when available, whereas Tree Swallows strongly relied on aquatic-emergent prey, regardless of the availability of wetlands in the immediate area around the nest (<500 m). Although wetland cover (within 1700 m) and row-crop cover (within 500 m), showed weak but negative effects on nestling body condition of Tree and Barn Swallows, respectively, nestling quality was strongly influenced by other variables such as nest and site. Using the Motus automated telemetry technology, I tracked early post-fledging apparent survival and movements of Barn and Tree Swallows. Estimates of post-fledging survival were similar between species, declining from 88.1% and 95.5% from tagging to fledging in Barn and Tree Swallows, to an apparent daily survival of 64.1% and 59% 8 days post-fledging and 26% and 30% 14 days after fledging, respectively. Apparent survival estimates were not related to individual factors such as body condition or age at fledging. Nevertheless, I obtained evidence that shorter development time in the nest, before reaching full locomotor development, may increase post-fledging mortality in Barn Swallows. Tree Swallow fledglings left their natal areas sooner than Barn Swallows and, after fledging, were found congregating at wetlands and marshes (> 5 km). This work adds to the growing evidence that conservation of wetlands and naturalized areas within the agricultural landscape in the Prairie Pothole Region is important for breeding aerial insectivores, including Tree and Barn Swallows, and may be helping mitigate the negative effects of intensive agricultural crop production. This study also contributes to filling gaps in our understanding of swallow ecology during the post-fledging period, a poorly understood stage of their life cycle and an important demographic driver of annual population growth in songbirds.

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