From city centre to urban edge: habitat heterogeneity and environmental filtering determine breeding bird community structure in an expanding Mediterranean city

Urban expansion alters animal communities through habitat fragmentation, environmental filtering and functional homogenisation. Understanding how urban habitats shape species assemblages is essential for urban planning that balances development with biodiversity conservation. We analysed the breeding bird community in Pamplona, northern Spain, a Mediterranean city that has experienced 40% population growth over the last five decades, examining how assemblages responded to habitat structure across the centre-periphery gradient. We detected 66 species organised according to the species-sorting paradigm, with two main Mediterranean assemblages: urban core specialists (Passer domesticus, Columba livia, Motacilla alba, Delichon urbicum) and peripheral species linked to natural habitats (Sylvia atricapilla, Troglodytes troglodytes, Luscinia megarhynchos, Hippolais polyglota). Water cover had the strongest positive effect on richness (+3.2 species per 10% habitat coverage), followed by gardens (+1.6 species per 10%), while building and street cover reduced richness (-1.0 to -1.5 species per 10%). Centre-periphery differences in species richness were measurable only at scales greater than 50 hectares, with 60 species in 120 ha in the urban periphery versus 40 in the centre, reflecting pronounced functional filtering favouring birds with preferences for wooded habitats and wetlands. Water bodies and well-wooded green areas functioned as biodiversity hotspots. The city's outskirts contributed to gamma diversity with rare species, highlighting the need for ecotonic and permeable urban boundaries, which should be established by landscape conservation strategies in expanding Mediterranean cities.