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. Selection through predation, snowfall, and microclimate on nest-site preferences in the Water Pipit Anthus spinoletta CLAUDIA M. RAUTER1,2,3, HEINZ-ULRICH REYER1 & KURT BOLLMANN1,4 1 Institute of Zoology, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland 2 Zoological Museum, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland 3 Present address: Department of Entomology, University of Kentucky, Lexington, KY 40546-0091, USA 4 Present address: Division of Biodiversity, Swiss Federal Institute WSL, Zuercherstrasse 111, CH-8903 Birmensdorf, Switzerland Word count: 6139 (text only) Address for correspondence: Heinz-Ulrich Reyer, University of Zurich, Institute of Zoology, Winterthurerstr. 190, CH-8057 Zurich, Switzerland Telephone: + + 41 1 635 4980 Facsimile: + + 41 1 635 6821 E-mail: [email protected] Rauter et al.: Selection for nest sites in Water Pipits page 2 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 slopes, where maximum temperatures were reached in the morning, nestling survival was higher than on WSW 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. The characteristics of a nest site can have a strong impact on reproductive success in birds (Collias & Collias 1984, Martin 1992). The characteristics of a nest site may reduce the risk of predation, but the specific nest site characteristics that improve safety can vary with the searching behaviour of predators (e.g. Martin 1988, Clark & Nudds 1991, Martin 1992, Hatchwell et al. 1999, Hooge et al. 1999). A nest site providing protection from inclement weather like heat waves, cold spells, rain storms, or snow may reduce losses of individual eggs or nestlings and the likelihood of complete nest failure (Lawton & Lawton 1980, Salzman 1982, Collias & Collias Rauter et al.: Selection for nest sites in Water Pipits page 3 1984, Penloup et al. 1997). Nest sites which improve the microclimate may provide thermal benefits to the eggs and nestlings (Webb & King 1983), reduce the energetic costs of nesting birds (Haftorn & Reinertsen 1985, Walsberg 1985, Chappell et al. 1990) and, hence, can result directly or indirectly in higher reproductive success (e.g. Austin 1976, Wachob 1996). An optimal nest site should provide protection from predators, shelter from inclement weather, and a favourable microclimate for eggs, young and incubating or brooding parents. However, the need for security, shelter and favourable microclimate can cause opposing selection pressures on nest placement (Lawton & Lawton 1980, Murphy 1983, Marzluff 1988, Halupka 1998). Knowing the relative importance of predation, weather, and microclimate in determining nest placement and the specific features of nest sites they favour is therefore basic to understanding how natural selection shapes the choice of nest sites in birds. Ground-nesting birds breeding at high elevations provide excellent opportunities to investigate these relationships. These birds face not only high predation pressure but also cold spells and snow storms which are not unlikely during the breeding season and can result in partial or complete nest failure (e.g. Verbeek 1970, Morton et al. 1972, King & Mewaldt 1987, Hendricks & Norment 1992, Bollmann et al. 1997). Furthermore, air temperature close to the ground regularly fluctuates between high and low extremes. During daytime, it can reach levels higher than the upper lethal temperature of eggs, while at night it may fall below 5°C or even below 0°C (Swan 1952, Hadley 1969, Franz 1979, Webb & King 1983), causing increased thermoregulatory costs for the adult bird (Haftorn & Reinertsen 1985). To investigate how predation, inclement weather and microclimate influence the placement of nests, we investigated their impact as agents of natural selection on nest site characteristics and its consequences for fitness correlates in Water Pipits Anthus spinoletta. Water Pipits, small insectivorous passerines, are common ground nesting birds in the Alps above the timberline (Glutz von Blotzheim & Bauer 1985). The females place their cup-shaped, grass, nests, into a hollow underneath a shrub, sod or rock. The hollow and the cover provided by the material surrounding it (roots of a Rauter et al.: Selection for nest sites in Water Pipits page 4 shrub, overhanging grass or soil) form a shelter for the nest (Böhm & Landmann 1995) and determine the microclimate in the nest cavity (Rauter & Reyer 2000). Snowfall during the Water Pipits’ breeding period is common, and one of the main causes of nest failure (Glutz von Blotzheim & Bauer 1985, Bollmann et al. 1997). Predation can be another important cause of partial or complete nest failure. At our study site, an Alpine valley, the main predators are Adders Vipera berus, Red Foxes Vulpes vulpes, Stoats Mustela erminea, and Carrion Crows Corvus corone (Bollmann et al. 1997). To investigate how predation by different types of predators (i.e. adders and birds/mammals), late snowfall, and microclimate might influence nest site selection in Water Pipits, we determine in this paper the relationship between these three selection factors, 12 nest site characteristics and reproductive success. We also consider the effects of food availability and feeding rates which, together with microclimate, affect the energy budget of the nestlings.