Effects of agricultural intensification on nestling condition and number of young fledged of barn swallows (Hirundo rustica).

Farmland bird populations have declined with increasing agricultural intensification possibly due to putative reductions in prey insects and effects of pesticide exposure. Presence of agriculture may be especially relevant for aerial insectivorous songbirds whose primary diet is flying insects. Here, we investigated the effects of agricultural land use on nestling body condition, an important determinant of post-fledging survival, for barn swallows (Hirundo rustica), an aerial insectivore breeding within an agro-ecosystem in southern Ontario, Canada. Our scale-of-effect analysis revealed that nestling and pre-fledging body condition varied most strongly with the proportion of row crop within 100 m of the natal barn. Unexpectedly, this correlation was positive for both nestling body condition (2016 only) and for pre-fledging condition (2016 and 2017). We found a weak positive effect of row crop on number of young fledged. We speculate that the positive effects of agricultural row-cropping on condition and number of young fledged was due to higher prey availability and/or more open foraging habitat around barns surrounded by row crops. Alternatively, higher nestling condition in high agriculture environments could reflect an insurance policy to increase survival during the post-fledging period. Our results suggest that, in our southern Ontario study area, the degree of agricultural conversion does not negatively influence individual nestling condition and number of young produced for barn swallows. We recommend future research on this species to examine reproductive success in more intense agricultural landscapes and possible effects of pesticide exposure.