Reduced breeding success in nest boxes for great tits (Parus major) and Eurasian blue tits (Cyanistes caeruleus) in highly urbanised areas: An ecological trap?

Nest boxes are widely installed in urban environments, yet the influence of their immediate habitat - particularly within cities - on bird reproductive success remains largely understudied. We investigated the reproductive outcomes associated with artificial nest box use by two secondary cavity-nesting species, the great tit (Parus major) and the blue tit (Cyanistes caeruleus), from 2019 to 2021 along an urbanisation gradient within the city of Dijon, France. We examined the impact of urbanisation on nest box occupancy, clutch size, hatching success, fledging success, and fledgling mass. Although pairs of great tits and blue tits did not avoid highly urbanised areas for nesting and laid larger clutches - indicating a higher initial breeding investment - their overall reproductive success was markedly lower in these sites. Indeed, both species experienced lower fledging success, and great tits additionally showed reduced fledgling mass in highly urbanised areas. Our findings suggest that nest boxes placed in these highly urbanised areas may function as ecological traps driven by maladaptive behaviours of breeding pairs and limited resource availability. These results highlight the need to carefully evaluate nest box placement for hole-nesting birds in urban landscapes, with broader implications for conservation efforts aimed at protecting sensitive species vulnerable to ecological traps.