Breeding success of tree swallows along a gradient of agricultural intensification

The intensification of agricultural practices has been identified as the main cause of population decline in farmland birds since the 1960s in both Europe and North America. Although the links between species richness or abundance and various components of agricultural intensi. cation are well established, the mechanisms underlying these trends have rarely been addressed along a gradient of intensi. cation or have been quanti. ed at only one spatial scale. Here we quanti. ed the in. uence of landscape structure on the nest box occupancy and breeding success of Tree Swallows (Tachycineta bicolor) at seven spatial scales (1, 2, 3, 4, 5, 10, and 20 km radii) over a 10 200- km 2 gradient of agricultural intensi. cation in southern Que ' bec, Canada. A network of 400 nest boxes distributed among 40 farms was visited every two days over three breeding seasons, 2004 - 2006. Nest box occupancy decreased with the proportion of intensive cultures ( maize, cereals, and soybeans) in the landscape, especially when manure heaps and tanks were abundant, and was also determined by local variables ( i. e., nest box clearance, interspeci. c competition) and by previous- year. edging success. Clutch size decreased as the breeding season progressed and with the proportion of intensive cultures in the landscape, with no consistent variation across spatial scales. Hatching success was not related to any landscape variables but increased with clutch size. Both the number of. edglings and. edging probability increased with the proportion of extensive cultures ( hay. elds, pastures, and fallows). These effects increased with spatial scale and reached a plateau at the 5 km radius: the maximum distance from the nest reached by foraging Tree Swallows. Our results can likely be attributed to lower food availability in intensive cultures compared to extensive ones. This study suggests that several components of breeding that impact on population structure and dynamics of insectivorous birds will be negatively affected by agricultural intensification.