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).