# Alpine songbirds at higher elevations are only raised with a slight delay and therefore under harsher environmental conditions

## TL;DR

Alpine songbirds breed at higher elevations with only a slight delay, facing harsher conditions compared to lower elevations.

## Contribution

This study reveals that songbirds at higher elevations breed with a smaller delay than expected, adapting to harsher conditions.

## Key findings

- Songbirds at higher elevations breed with only a 0.5–5.4 day delay compared to lower elevations.
- Climatic delays (34–38 days) are much greater than breeding delays observed in birds.
- Birds may have evolved adaptations to cope with harsher high-elevation conditions rather than delaying breeding.

## Abstract

The breeding phenology of birds is often timed to coincide with a peak in food availability. However, the shortening of the vegetation period with increasing elevation may force bird species at high elevations to breed earlier in relation to optimal environmental conditions due to time constraints. We investigated differences in fledging dates in five Alpine woodland songbird species along an elevational gradient from 1500 to 2200 m in Switzerland. We estimated fledging dates from a nationwide citizen science bird monitoring dataset and used the date when the proportion of observations of ‘fledged young’ reached 50% among all observations indicating breeding behaviour. This measure had the advantage that we could estimate average timing of the broods across a wide geographic range and over many years without the need to search for individual nests. We then compared differences in timing of the broods with climatic conditions and larch budburst across different elevational bands. The daily mean air temperature of 10–15°C was reached 34–38 days later at 2200 m compared to 1500 m, which is a similar delay as found in previous reports on snow melt‐out date. The average delay in larch budburst was 19.2 days at 2200 m compared to 1500 m. In comparison, the average timing of the birds' broods was only 5.4 days later in coal tits and 0.5 days later in Alpine tits at 2200 compared to 1500 m (the two species for which we had the narrowest interval estimates). Also, the estimated delay at higher elevations in the broods of song thrushes, mistle thrushes and Eurasian chaffinches was relatively small. Rather than postponing breeding dates to better environmental conditions later in the season that would match the earlier conditions at low elevation, songbirds breeding at higher elevations may thus have evolved adaptations to cope with the harsher conditions.

Climatic conditions and food availability change with increasing elevation, thus breeding phenology of song birds may be delayed at higher compared to lower elevation. To investigate differences in average timing of broods along an elevational gradient, we used data from a nationwide citizen science bird monitoring scheme within the Swiss Alps. The investigated bird species raised their young with a smaller delay at higher elevation than expected, thus under harsher environmental conditions than at lower elevations.