2.3. Bird survey and bird traits
Bird surveys were conducted by a single observer (anon.) using 10-min point counts (Bibby et al., 2000). Points were visited twice per year (late March-early April and late April-late May) in two consecutive years (20021-22) during the early morning under calm and dry weather conditions. To avoid possible bias due to variation in diurnal activity of birds, points were visited in a different order during the second survey period, leaving at least three weeks between consecutive visits to the same point. All birds contacted, visually and acoustically, were recorded within five distance bands of increasing radius from the centre of the point (0–25 m, 25–50 m, 50–75 m, 75–100 m and >100 m). We later excluded birds flying over (e.g. swallows, swifts Apus sp.), aquatic birds (i.e. Ardeidae species), raptors and migrating species from the analysis that were inadequately sampled by our approach (see Supporting Information 1) (Marcolin et al., 2021).
Species were classified as native or alien, the latter being species whose native range is outside Europe and for whom their provenance in Europe is known or very strongly suspected to be through deliberate or accidental introduction by humans (5 species, see Supporting Information 1). The exception was Feral Pigeon Columbia livia var.domestica whose status is unclear. Although this species derives from the wild Rock Dove, a declining species which still has native populations in Europe, the long history of domestication of this species has led some to classify it as alien (Boano et al. 2019, Lowther & Johnston, 2020). Given this and the often very significant numbers of Feral Pigeon, we carried out the main analyses considering it both as a native and as an alien species.
We classified the surveyed bird species using a set of 10 functional traits that reflected resource-use of individuals (Flynn et al., 2009) and resource overlap between species (Andrikou-Charitidou et al., 2020), and that influence species response ability to habitat changes (Anderle et al., 2022): i) clutch size; ii) number of broods per year; iii) body mass; iv) migratory status; v) territoriality; vi) breeding season habitat use; vii) breeding season diet; viii) foraging stratum; ix) foraging technique; and, x) nesting habit (see Supporting Information 1 for a detailed description and data sources). Apart from clutch size, number of broods per year and body mass, the categories from the other traits were translated into binary variables (i.e. each trait was either present or absent for a given species; Supporting Information 1). To account for the different scales of continuous traits, clutch size, broods per year and body mass (previously log transformed) were scaled to values between 0 and 1. There was no marked intercorrelation between trait variables (all Pearson’s r < 0.7).
To test the effect of alien bird species presence on the diversity metrics of bird communities, we followed Loiola et al. (2018): community type was defined for each point as invaded when at least one alien species was found in at least one visit (non-invaded vsinvaded communities). Moreover, to study the contribution of alien species to the diversity metrics of the native component of a community, we considered a third community type, that of the native species in invaded communities, i.e. excluding alien species from the invaded communities (invaded no alien ; Supporting Information 1).