Data and code from: Large-scale source-sink dynamics in Tachycineta bicolor breeding in nest-boxes across eastern Canada's farmlands

Many wild species are declining due to human driven environmental changes, including landscape transformation due to agricultural intensification. This is especially true for insects and their avian predators worldwide. To understand how populations respond to human-induced environmental changes, detailed data on survival, reproduction and dispersal are essential especially in declining populations where immigration can buffer local declines. We used multi-state Capture-Mark-Recapture (CMR) models and 14 years of data (2006-2019) on individually marked Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) breeding on farmlands in southern Québec, Canada, to quantify the effects of agricultural intensity, local weather, and interspecific competition on vital rates and population growth. Annual population growth rates were strongly influenced by immigration and adult return rates. Nest site competition from House Sparrows (Passer domesticus (Linnaeus, 1758)), primarily present in intensively cultivated landscapes, was the main factor driving the observed decline in productivity and adult return rates. Our results suggest that intensively cultivated farmlands act as population sinks, making them unsustainable for Tree Swallows without immigration. Our findings highlight the importance of considering both landscape context and population connectivity when assessing demographic processes. They also underscore the need to study multiple populations over large geographic regions to identify drivers of large-scale declines.