Selection through predation, snowfall and microclimate on nest-site preferences in the Water Pipit Anthus spinoletta

Nest-site characteristics can have a strong impact on reproductive success in birds. Nest sites should simultaneously protect from predators, offer shelter and provide a favourable microclimate. We studied the relationship between three agents of natural selection (predators [i.e. Adders and birds/mammals], snowfall and microclimate), nest-site characteristics and reproductive success to determine whether these influenced preference for specific nest-site characteristics in the Water Pipit Anthus spinoletta. Pooled over all nests, the relative importance as agents of natural selection decreased from mammalian/avian predation (15% of all nests) through Adder predation (12%) to snowfall (7%), but there were clear differences in space and time. Predation by Adders selected for nest sites surrounded by few medium-sized shrubs. Selection by mammalian and avian predators favoured no specific nest-site characteristics. Protection from snowfall was best in nests surrounded by relatively few medium-sized shrubs. Microclimate had a strong influence on nestling survival and duration of nestling period. In nests on ENE-facing slopes, where maximum temperatures were reached in the morning, nestling survival was higher than on WSW-facing slopes, where temperature maxima occurred in the afternoon. Our results indicate that weak, but significant, directional selection is acting on preference for certain nest-site characteristics through effects on survival and development of nestlings. As predation and snowfall are unpredictable, the evolution of an optimal nest placement strategy is unlikely on a small scale. On a larger scale, however, choice of one breeding area over another may be favoured because of predictable differences between locations in terms of survival and nestling development.