Size doesn’t always matter: Greenspace connectivity can offset insufficient habitat patch size to improve urban tits breeding success

Urban landscapes are often highly fragmented, constraining animals to live in and exploit a multitude of habitat patches (e.g., greenspaces) of varying size and isolation. Small greenspaces may not contain enough resources for species to maintain viable populations. Yet, appropriate spatial configuration of the habitat network (i.e., high greenspace connectivity) could theoretically alleviate or even compensate the local food resource limitations by allowing access to additional foraging grounds. Surprisingly, this effect has never been tested to explain the reproductive performances of urban fauna. We hypothesised that higher greenspace connectivity would improve the breeding outputs of two insectivorous bird species (tits), especially with decreasing nesting greenspace area (i.e., the habitat patch where the nest is located). For four years, we monitored the survival and mass of nestlings of Parus major and Cyanistes caeruleus using 240 nestboxes located along a multivariate urban gradient in Dijon (France), and analysed their variations with GLMMs while controlling for various confounding factors (e.g., urbanness, noise and light pollution, microclimate, vegetation management). Functional connectivity was measured through graph-based modelling. Greenspace connectivity was important to explain both nestling survival and mass while the nesting greenspace area was not a good predictor of breeding success. Furthermore, the positive effect of connectivity on nestling survival significantly increased with decreasing area of their nesting patches. Urban tits can maintain successful reproduction dynamics in small greenspaces provided those are well connected to the network of preferred foraging habitats. Promoting the connectivity of interstitial urban greenspaces could thus partly compensate for their frequently insufficient sizes in cities.