Combined influence of food availability and agricultural intensification on a declining aerial insectivore

Abstract

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 effects aerial insectivore fitness. The relationship between prey availability and the fitness of insectivores may thus vary with the extent of agricultural production. 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 Qu&eacutebec, Canada. More specifically, we evaluated the landscape characteristics affecting prey availability, and how this relationship influences the fledging success, the duration of the nestling period, the 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, yet within principally agricultural landscapes, it was greatest within ones dominated by intensive row crops (corn, soybean and wheat), and, counter to our predictions, lowest within those dominated by forage and pasture fields. Of the measured fitness proxies, only fledging success was 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, once conditioning on 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.

Interacting effects of cold snaps, rain, and agriculture on the fledging success of a declining aerial insectivore

“…Traps were spaced ∼ 250 m apart along the central portion of each nest box transect. Trap content was collected every two days throughout each breeding season (see Garrett et al 2021a for details). The processing of insect samples focused on the period between 1 June and 15 July, covering the nestling rearing period of 96% of the monitored breeding attempts.…”

“…We compared a set of competing models, including a Null model with only random effects. These data were previously used to determine the combined influence of prey availability and landscape context within our study area on fledging success, and as a result a most predictive and parsimonious model had then been determined (Garrett et al 2021a). We thus compared all subsequent models to this base model (Base).…”

“…Traps were spaced ~ 250 m apart along the central portion of each nest box transect. Trap content was collected every two days throughout each breeding season (see Garrett et al 2021a for details).…”

“…The relative cover of this habitat within 500 m of nest boxes (0.66% ± 1.07%) and in the agricultural contexts covered by this study is low. Recognizing this habitat as important to Tree Swallows (Berzins et al 2021), we calculated its relative cover at 3 km from nest boxes following methods presented in Garrett et al (2021a). Calculations used vector layers acquired from the Canadian National Hydro Network (NHN, 2020) and the sf (Pebesma 2018) and rgeos (Bivand and Rundel 2019) packages in R version 3.6.2 (R Core Team 2019).…”

“…Certain agricultural practices are observed to alter insect populations and communities through changes in abundance, phenology, or species composition and interactions (Grüebler et al 2008, Pisa et al 2015, Wagner 2020). Insectivorous birds breeding in modern agricultural landscapes, including aerial insectivores, could thereby be subjected to reduced availability of prey items (Poulin et al 2010, Nocera et al 2012, Garrett et al 2021a).…”