Timing reproduction to coincide with optimal environmental conditions is key for many organisms living in seasonal habitats. Advance in the onset of spring is a particular challenge to migratory birds that must time their arrival without knowing the conditions on the breeding grounds. This is amplified at high elevations where resource availability, which is linked to snowmelt and vegetation development, shows much annual variation. With the aim of exploring the effects of variability in the onset of local resource availability on reproduction, we compared key life history events in an Alpine population of the Northern Wheatear (Oenanthe oenanthe) between years of contrasting timing of snowmelt. Based on remote sensed images, we identified 2020 as an exceptionally early snowmelt and green-up year compared to the preceding year and the long-term average. Individuals tracked with light-level geolocators arrived well before the snowmelt in 2020 and clutch initiation dates across the population were earlier in 2020 compared to 2019. However, observations from a citizen science database and nest monitoring data showed that the arrival-breeding interval was shorter in 2020, thus the advance in timing lagged behind the environmental conditions. While hatching success was similar in both years, fledging success was significantly reduced in 2020. A trophic mismatch in early 2020 could be a possible explanation for the reduced reproductive success, but alternative explanations cannot be excluded. Our results show that, despite the timely arrival at the breeding grounds and a contraction of the arrival-breeding interval, Wheatears were not able to advance breeding activities in synchrony with environmental conditions in 2020. Earlier reproductive seasons are expected to become more frequent in the future. We show that the negative effects of changing seasons in Alpine migratory birds might be similar to birds breeding at high latitudes, despite their shorter migratory distance.

Climate change is leading to the advancement of spring conditions, resulting in an earlier snowmelt and green-up, with highest rates of change in highly seasonal environments, including alpine habitats. Migratory birds breeding at high elevations need to time their arrival and lay dates accurately with this advancement, but also with the annually variable spring conditions at their breeding sites, to maximize nest survival probabilities and reproductive output. Nest survival probability and mean nestling mass were analysed in relation to lay date and habitat conditions in an alpine population of the migratory Northern Wheatear Oenanthe oenanthe collected over six consecutive breeding seasons in the Western Italian Alps. This open grassland species showed the lowest nest survival probability in years with an early onset of spring conditions. Within-season, nest survival was highest when breeding late, at lower elevations, and when grass cover and grass height were higher. Both across-and within-season, severe weather conditions may indirectly lead to higher early season nest failure rates by increasing predation risk. By contrast, mean nestling mass, and thus the quality of the fledglings, was lower when breeding late. This might be driven by a mismatch with the peak in food abundance. Breeding early is thus generally advantageous in terms of chick quality in our highelevation population, but reproductive success is limited by the risk of nest failure that is higher in early springs and early in the season. This trade-off between breeding early and late may thus allow Northern Wheatears to maximize fitness under highly variable spring conditions. However, climate change may cause disruption to this trade-off, and shifts in phenology could become a threat for migratory alpine birds that might not be able to keep track of advancing spring conditions.

A steady advance in the onset of spring is one of the most prominent footprints of climate warming and requires organisms, including migratory birds, to adapt their annual routines. As lower trophic levels typically adapt faster than higher trophic levels, observations of reduced fitness due to trophic mismatches are becoming more frequent, especially in long-distance migratory birds. We aimed to identify key phenological events, and quantify potential mismatches and their consequences in a migratory songbird population of the Northern wheatear (Oenanthe oenanthe) breeding at high elevations in the European Alps.We used light-level geolocators to track wheatears, and collected information on individual breeding activity and breeding success as well as environmental conditions during the reproductive season. In addition, we used citizen science data and remote sensed images to quantify longer term phenological trends.Snow melt and green-up showed an exceptionally early spring in the study region in 2020, preceded by a relatively average year in 2019. Yet, tracked individuals arrived well before the snowmelt in 2020 and clutch initiation dates across the population were earlier in 2020 compared to 2019. However, this shift lagged behind the advance in environmental conditions. While hatching success was similar in both years, fledging success and overall nest success was significantly reduced in 2020.Our results show that, despite the timely arrival at the breeding grounds, wheatears did not advance breeding activities in synchrony with environmental conditions during the exceptionally early year in 2020. The reduced fledging success suggests a trophic mismatch. However, the underlying mechanisms for hatchling mortality and nest failure remain unknown. Earlier reproductive seasons are expected to become more frequent in the future. We show that the negative effects of changing seasons in Alpine migratory birds might be similar to birds breeding at high latitudes, despite their shorter migratory distance.

Mountains support high levels of biodiversity, but they are also particularly vulnerable to climate change. Whilst studies on mountain biodiversity at the species level are common, studies that consider whole assemblages are scarce. We assessed how an alpine bird assemblage varied in terms of ecological habitat niche by surveying bird communities and habitat at point counts placed along elevation gradients in the Western Italian Alps. Niche breadth as measured by habitat use increased along the gradient, suggesting that being more generalist is an advantage in terms of survival at higher elevation. Niche position also increased with elevation, which means that species occurring at higher elevations use habitats that are atypical with respect to the average species in the community. Both niche breadth and position were negatively associated with habitat diversity, but these relationships were mainly driven by the species occurring at the very highest elevations (>2500 m), suggesting that true alpine specialist show a different pattern from the other species of the assemblage. Our results therefore generally supported the idea that having a wider niche breadth is useful in harsh environments, such as mountains, enabling the exploitation of a wider range of resources. The broader niche of many high elevation species may therefore indicate some degree of resilience to environmental change, as long as key habitat types are maintained.

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