Different traits shape winners and losers in urban bird assemblages across seasons

Abstract

Urbanisation is a major driver of global biodiversity decline, profoundly affecting animal communities. While most studies on bird communities have primarily focused on the breeding season, we aimed to identify species responses and their associated traits by adopting a stratified design and using a multi-season approach considering a gradient from highly urbanised city centres to the urban-rural fringe across six Italian cities. We found that bird assemblages exhibited different responses to urbanisation according to season. Winners (i.e. species positively affected by urbanisation) were characterised by traits such as colonial nesting, high productivity and longevity. In winter, these species displayed generalist foraging strategies and solitary behaviour. Losers (i.e. species negatively affected by urbanisation) tended to be insectivorous, ground-nesting and short-distance migratory species. Interestingly, intra-specific variations emerged, with wintering populations of some species exploiting highly urbanised areas despite not breeding there. Urban adapters, although not strictly winners, displayed resilience by navigating a range of urban conditions, effectively exploiting intermediate levels of urbanisation. This study provides novel insights into the complex ecological dynamics occurring within the urban matrix in different seasons. Our findings emphasise the importance of adopting a multi-season approach in research and urban planning to better understand species responses and develop more effective, sustainable strategies for biodiversity conservation in urban environments.

Keywords: Community ecology; Green cities; Green spaces; Imperviousness; Urban biodiversity; Urban gradient.

Fig. 1

Study design and details on sampling methodology. Location of the six cities surveyed in the Italian peninsula (A), details of the study design within the city of Rome, as an example (B) and point counts (black dots) and the 100 m buffer (white circle) with land use (C). Map designed in QGIS Software (datum and projection: EPSG:3003—Monte Mario/Italy zone 1).

Fig. 2

Regression plots from the species-specific GLMMs with occurrence probability in relation to the imperviousness gradient across seasons for winners (A) and losers (B). Point identity and city were added as nested random factors in the models. Winners were classified based on significantly positive linear responses to urbanisation and losers based on significantly negative responses. For black redstart (Phoenicurus ochruros) a significant (p < 0.05) interaction between imperviousness and season was highlighted. See Table S2 for results.

Fig. 3

Regression plots from the species-specific GLMMs with occurrence probability in relation to the imperviousness gradient across seasons for urban adapters. Point identity and city were added as nested random factors in the models. Urban adapters were classified based on non-significant or significantly non-linear responses to urbanisation. For blue tit (Cyanistes caeruleus), blackcap (Sylvia atricapilla), Sardinian warbler (Curruca melanocephala) and blackbird (Turdus merula) a significant (p < 0.05) interaction between imperviousness and season was highlighted. See Table S2 for results.

Fig. 4

Heatmaps of the relationships between bird traits and the imperviousness gradient for the breeding and wintering seasons derived from the fourth-corner analysis with a LASSO penalty. White cells represent non-significant associations whereas darker colours represent stronger associations, with blue representing positive correlations (i.e. traits associated with highly impervious areas) and red negative ones (i.e. traits associated with green spaces). ADN is the association degree during nesting whilst AOBS is the association degree outside the breeding season. See Table 1 for further details on traits.