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 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

Negative effects of agricultural intensification on the food provisioning rate of a declining aerial insectivore

“…Residual woodlots within these agro-intensive landscapes add to the tree lines separating fields to increase the length of "forest" edges, where insect abundance tends to be greatest (Boetzl et al, 2020;Evans et al, 2003;Jokimäki et al, 1998). The fledging success of Tree Swallows from the system presented here is greater in agro-intensive landscapes containing residual woodlots than in ones denuded of forest cover (Garrett et al, 2022a). It may therefore be hypothesized that the elevated insect abundance associated with these residual woodlots lead to greater provisioning rates and thus greater fledging success in this landscape context.…”

“…Previous studies have indeed observed that agricultural intensification, including habitats impactful to insect communities and abundances, correlate with changes in dietary composition (Bellavance et al, 2018;English et al, 2017;Nocera et al, 2012), reductions in annual fitness proxies (Benton et al, 2002;Garrett et al, 2022a;Ghilain & Bélisle, 2008), and altered population growth (Stanton et al, 2018) of several aerial insectivore species. Studies linking prey availability to aerial insectivore breeding success have, however, often failed to observe such relationships (Dawson & Bortolotti, 2000;Dunn et al, 2011;Imlay et al, 2017;McClenaghan et al, 2019).…”

“…Confounding factors included the brood demand proxy as a third-order polynomial, as well as the percent cover of surface water within 1 and 10 km and their interaction. We further included proxies of local prey availability, including the ordinal date and the maximum observed temperature during each 1-h sampling period as a second-order polynomial (Bellavance et al, 2018, Garrett et al, 2022aRioux Paquette et al, 2013). Though the principal model of interests was one including site scores and their interaction (Base + Land), we compared another three models hypothesizing that the effects of foraging within agro-intensive landscapes may either be alleviated or exacerbated by confounding variables.…”

“…Our measure of food availability was based on sampling over a 2-day period prior to nest box visits, while our sampling of food provisioning occurs over a 1-h period. We believe that, despite food availability being linked to the fitness of Tree Swallows in this system (Garrett et al, 2022a), the differing temporal scales of sampling likely hinders our ability to confidently evaluate how local food availability is related to food provisioning. Instead, we suggest future work explicitly focus on sampling aerial insects during food provisioning sessions, as has been used in other similar studies (Elgin et al, 2020;Grüebler et al, 2008;Stanton et al, 2016).…”

“…A recent 11-year study in southern Québec, Canada, suggested that landscapes dominated by more arable row-crop monocultures (e.g., corn [Zea mays] or soybean [Glycine max]) offer significantly less aerial insectivore prey than ones dominated by less agriculturally intensive habitats such as forage crops, grasslands, and dairy farms (Garrett et al, 2022a). Furthermore, in this same study, and upon controlling for local prey availability, several indices of Tree Swallow (Tachycineta bicolor) breeding success remained associated with an agricultural gradient (Garrett et al, 2022a). Breeding attempts within landscapes dominated by intensively managed row crops resulted in reduced fledging success, longer nestling periods, and shorter fledging wing lengths.…”