Dataset for: Assessing pesticides exposure effects on the reproductive performance of a declining aerial insectivore

In the context of increasing global environmental changes, it has become progressively important to understand the effects of human activity on wildlife populations. Declines in several avian populations have been observed since the 1970s, especially with respect to many farmland and grassland birds, which also include some aerial insectivores. Changes in farming practices referred to as agricultural intensification coincide with these major avian declines. Among those practices, increased pesticide use is hypothesized to be a key driver of avian population declines as it can lead to both toxicological and trophic effects. While numerous laboratory studies report that birds experience acute and chronic effects upon consuming pesticide treated food, little is known about the effects of the exposure to multiple pesticides on wildlife in natural settings. We monitored the breeding activities of tree swallows (Tachycineta bicolor) on 40 farms distributed over a gradient of agricultural intensification in southern Québec, Canada, to evaluate the presence of pesticides in their diet and quantify the exposure effects of those compounds on their reproductive performance between 2013 and 2018. We first assessed the presence of 54 active agents (or derivatives) found in pesticides in 2,081 food boluses (insects) delivered to nestlings by parents and documented their spatial distribution within our study area. Second, we assessed the effect of pesticide exposure through food (number of active agents detected and number of contaminated boluses on a given farm for a given year, while controlling for sampling effort) on clutch size as well as hatching and fledging successes and nestling's mass upon fledging. Pesticides were ubiquitous in our study system and nearly half (46%) of food boluses were contaminated by at least one active agent. Yet we found no relationship between our proxies of food contamination by pesticides and tree swallow reproductive performance. More studies are needed to better understand the putative role of pesticides in the decline of farmland birds and aerial insectivores as potential sublethal effects of pesticides can carry over later life stages and impact fitness.Variables description for DB_females yearfarm: concatenante of year and farm number yearidnestbox: concatenate of farm and nestbox id idnestbox: concatenante of farm and nestbox number neggs: number of eggs laid nhatchlings: number of nestlings that hatched nfledglings: number of nestlings that fledged ndead: number of nestlings that died before fledging dlaying: julian day of first egg laid dincub: julian date of incubation start dhachingmin: minimum julian date of hatching dhatchingmax: maximum julian date of hatching dflegingmin: minimum julian date day of fledging dfledgingmax: maximum julian date of fledging idfemale: unique female id mass: female mass (g) age_morpho: female morphological age d12: julian date with clutch aged 12 days year: year farm: farm id nonintensive: % of non-intensive culture within 500-m of the nest box intensive: % of intensive culture within 500-m of the nest box water: % of water cover within 500-m of the nest box npestdiff.54: number of different pesticides detected for a specific farm-year nboul2tot: total number of boluses collected for a specific farm-year nboulcont.54: number of contaminated boluses for a specific farm-year nnonfledging: number of nestlings that didn't fledged (includes dead and disapeared nestlings) hatchingsuccess: number of hatchlings divided by number of eggs laid fledgingsuccess: number of fledglings divided by number of hatchlings Variables description for DB_nestlings idclutch: concatenate of nest box id, year and clutch number yearfarmnich: concatenante of year, farm and nest box id idfemale: unique female id idnestling: unique nestling id nestling_massd16: nestling's mass at 16 days old (g) fledged: 0 if nestling did not flegde, 1 if it fledged age_morpho: female morphological age female_mass: female mass (g) nonintensive: % of non-intensive culture within 500-m of the nest box nboul2tot: total number of boluses collected for a specific farm-year nboulcont.54: number of contaminated boluses for a specific farm-year year: year dlaying: julian day of first egg laid idnestbox: concatenante of farm and nestbox numbe