Species Traits Drive Long-Term Population Trends of Common Breeding Birds in Northern Italy

Abstract

Long-term population trends are considerable sources of information to set wildlife conservation priorities and to evaluate the performance of management actions. In addition, trends observed in functional groups (e.g., trophic guilds) can provide the foundation to test specific hypotheses about the drivers of the observed population dynamics. The aims of this study were to assess population trends of breeding birds in Lombardy (N Italy) from 1992 to 2019 and to explore the relationships between trends and species sharing similar ecological and life history traits. Trends were quantified and tested for significance by weighted linear regression models and using yearly population indices (median and 95% confidence interval) predicted through generalized additive models. Results showed that 45% of the species increased, 24% decreased, and 31% showed non-significant trends. Life history traits analyses revealed a general decrease of migrants, of species with short incubation period and of species with high annual fecundity. Ecological traits analyses showed that plant-eaters and species feeding on invertebrates, farmland birds, and ground-nesters declined, while woodland birds increased. Further studies should focus on investigation of the relationship between long-term trends and species traits at large spatial scales, and on quantifying the effects of specific drivers across multiple functional groups.

New Evidence on the Linkage of Population Trends and Species Traits to Long-Term Niche Changes

“…Compared to the Spearman rho statistic, Kendall tau provides more accurate p-values when the sample dimension is small and there are many ties in the data [86], as in our case. We derived species trends from [53], where long-term trends (1992-2019) for the species considered in this study were assessed in the same study area, using data derived from the same bird monitoring program. We assigned a zero value to species with non-significant trends.…”

“…To investigate the relationship between niche metrics (Schoener's D, expansion and unfilling index) and species traits, we performed distinct analyses for continuous traits (12 life-history traits, namely body length, wing length, tail length, bill length, tarsus length, body weight, clutch size, incubation period, fledging period [87]; number of broods per breeding season [53,87]; annual fecundity [88]; dispersal ratio [89,90]; and six species specialization indices, namely foraging habitat, acquisition behavior, nesting habitat, foraging substrate, diet [53,91,92]; overall [53]) and categorical traits (four uncorrelated traits of interest [53], namely migration strategy [53], nest type [53,87], landscape type-i.e., habitat preferences at landscape scales [53]-, and diet [53,93]). For continuous traits, we performed a principal component analysis (PCA) using all 18 traits as input variables.…”

“…Beyond the relevance of a species-specific approach, in the past decade the increasing attention on a trait-based approach allowed highlighting emergent ecological patterns in groups of birds sharing similar life-history and ecological traits [49][50][51][52][53]. The relationship between the ecological niche and species traits has been mainly explored under an evolutionary perspective ( [54][55][56]), but there is a lack of studies focusing on ecological aspects (but see [57,58]).…”

“…by a restricted pool of expert surveyors, thus limiting the presence of inter-observer detection bias. For the purpose of this study, we retained the most common species [53] with a frequency of occurrence higher than or equal to 1% in both T1 and T2 (number of species = 71).…”

Drivers of migrant passerine composition at stopover islands in the western Mediterranean

“…An alternative is that differences in temporal variability originate from differences in species composition. Given that continental abundance determines to a large extent migrant abundance, if populations of some species are more variable interannually 48 – 50 and these species are more abundant on some islands, those islands would present more variation in migrant composition between years. In addition, some species could be more sensitive than others to variation in meteorological conditions 51 , 52 which could cause more interannual variability in migrant composition.…”