Costs and benefits of nest cover for ptarmigan: changes within and between years.

Selecting an 'optimal' nest site is believed to be an important component of fitness for birds, but paradoxically there is often considerable intraspecific variation in nest sites. Few studies have tested whether nest-site selection by birds changes after experience with nest predators. Using data collected over 9 years, we examined within- and between-year variation in 331 nests of white-tailed ptarmigan, Lagopus leucurus. We found the first direct evidence for a fitness trade-off with amount of cover at the nest; concealed sites were less often detected by predators but were more risky for incubating hens. Small ptarmigan and those in good body condition used sites with less cover, while large ptarmigan nested at higher elevations. Nesting success was not related to age or spring body condition. Responses to experience with predators were less clear. Successful females tended to use less cover the following year, whereas unsuccessful females tended to change cover types, but not the amount of cover, in subsequent years. Overall, characteristics of nest sites showed strong seasonal patterns, suggesting that microclimate may be a more important selective pressure than predation. Copyright 1998 The Association for the Study of Animal Behaviour.