Nestling diet and parental food provisioning in a declining mountain passerine reveal high sensitivity to climate change

Abstract

Mountain ecosystems naturally experience strong seasonal weather variations leading to a brief peak in food availability that constrains bird reproduction. Climate change accentuates both the intra‐ and interannual weather variability, which in turn can reduce the predictability of food resources and hence impact population demography. Yet, relatively little is known about the influence of environmental factors on the breeding ecology of mountain birds. Here, we quantified the nestling diet and provisioning behaviour of the Alpine ring ouzel Turdus torquatus alpestris, an emblematic and declining thrush species typical of central European treeline ecotones, and relate these parameters to local weather conditions. Nests were monitored with camcorders to assess prey provisioning frequency and identify items delivered by parents to nestlings, as well as to estimate prey biomass. Our results indicate the prominence of earthworms (Lumbricidae) in the nestling diet, both in terms of abundance (80%) and biomass (90%). Elevated ambient temperatures negatively impacted both prey provisioning rates and biomass delivered to chicks by parents, while rainfall had a positive effect on the delivered biomass. The mean prey item biomass decreased throughout the breeding season, as did the proportion of earthworms in nestlings' diet. These findings highlight the key role played by local weather in parental provisioning behaviour, probably reflecting the low availability of the staple food source, earthworms, in warm and dry weather contexts. In particular, they underpin how climate alterations, notably increasing ambient temperatures and changing precipitation regimes, could impact mountain birds. Although effects on reproductive performance and population dynamics still ought to be studied, these results further our understanding of the ecological mechanisms potentially at play in the decline of wildlife inhabiting high‐elevation, climate‐sensitive ecosystems.

Early Breeding Conditions Followed by Reduced Breeding Success Despite Timely Arrival in an Alpine Migratory Songbird

“…While this might point toward phenological asynchrony, since the differences occurred during the most resource-demanding period and the peak in NDVI was missed by about a week, we can only speculate about the underlying mechanisms of hatchling mortality. To identify food shortages, data on chick growth and prey abundance (Barras et al, 2021) are necessary. However, phenological asynchronies may also have indirect effects.…”

“…Clutches experiencing a rainy season or severe weather events, such as snow storms, have a prolonged incubation period, exposing the nest to predators for a longer period at the same time, and generally higher failure rates (due to abandonment, hatch failure and predation) than clutches in a dry season (Rauter et al, 2002;Kluen et al, 2011;Higgot et al, 2020;Marcelino et al, 2020). Further explanations for variations in breeding success in alpine birds include annual variations in temperature, where success is lower in years with lower mean temperatures (Mingozzi et al, 2021), but also variations in weather-dependent resources (Barras et al, 2021) and local predation risk (Rauter et al, 2002).…”

Predictive models of distribution and abundance of a threatened mountain species show that impacts of climate change overrule those of land use change

“…In Switzerland, its breeding population has been declining by more than one-third in the 30 last years, mostly at lower elevations, resulting in a mean upward distribution shift of +84 m (Knaus et al, 2018). This strongly suggests that climate change, with a linear mean increase in ambient temperature of 0.9°C over the same period (Meteoswiss, 2019), might be responsible for the decline (Barras et al, 2021;Beale et al, 2006). Nevertheless, this climatic trend has been paralleled by accelerating land abandonment, which is the main driver of forest cover increase and upward shift in the country (Gehrig-Fasel et al, 2007), with a 46,200 ha wooded area gain in the Swiss Alps from 1985 to 2009 (SFSO, 2013).…”

Complex Day-to-day Movements of an Alpine Passerine May Act as an Insurance Against Environmental Variability

“…Later in the season, with the advancement of the snowmelt which frees the rst patches of alpine grasslands within the breeding area, they stopped commuting. Daily elevational migration may enable birds to settle in highelevation breeding grounds very early in the year, sometimes when those are still inhospitable, and thus some sort of insurance against missing the brief time window suitable for breeding (Barras et al 2020;Barras et al 2021). Indeed, ring ouzels migrate much faster in the spring than in the autumn, a pattern commonly observed across various migration yways and species (Nilsson et al 2013;Schmaljohann 2018).…”