Data set for combined influence of food availability and agricultural intensification on a declining aerial insectivore

Aerial insectivores show worldwide population declines coinciding with
shifts in agricultural practices. Increasing reliance on certain
agricultural practices is thought to have led to an overall reduction in
insect abundance that negatively affects aerial insectivore fitness. The
relationship between prey availability and the fitness of insectivores may
thus vary with the extent of agricultural intensity. It is therefore
imperative to quantify the strength and direction of these associations.
Here we used data from an 11-year study monitoring the breeding of Tree
Swallows (Tachycineta bicolor) and the availability of Diptera (their main
prey) across a gradient of agricultural intensification in southern
Québec, Canada. This gradient was characterized by a shift in agricultural
production, whereby landscapes composed of forage and pastures represented
less agro-intensive landscapes and those focusing on large-scale arable
row crop monocultures, such as corn (Zea mays) or soybean (Glycine max)
that are innately associated with significant mechanization and
agro-chemical inputs, represented more agro-intensive landscapes. We
evaluated the landscape characteristics affecting prey availability, and
how this relationship influences the fledging success, duration of the
nestling period, fledgling body mass, and wing length as these variables
are known to influence the population dynamics of this species. Diptera
availability was greatest within predominately forested landscapes, while
within landscapes dominated by agriculture, it was marginally greater in
less agro-intensive areas. Of the measured fitness and body condition
proxies, both fledging success and nestling body mass were positively
related to prey availability. The impact of prey availability varied
across the agricultural gradient as fledging success improved with
increasing prey levels within forage landscapes yet declined in more
agro-intensive landscapes. Finally, after accounting for prey
availability, fledging success was lowest, nestling periods were the
longest, and wing length of fledglings were shortest within more
agro-intensive landscapes. Our results highlight the interacting roles
that aerial insect availability and agricultural intensification have on
the fitness of aerial insectivores, and by extension how food availability
may interact with other aspects of breeding habitats to influence the
population dynamics of predators.