COSTS AND BENEFITS OF NEST COVER FOR PTARMIGAN - CHANGES WITHIN AND BETWEEN YEARS

Selecting an 'optimal' nest site is believed to be an important compon ent of fitness for birds, but paradoxically there is often considerabl e intraspecific variation in nest sites. Few Studies have tested wheth er nest-site selection by birds changes after experience with nest pre dators. Using data collected over 9 years, we examined within- and bet ween-year variation in 331 nests of white-tailed ptarmigan, Lagopus le ucurus. We found the first direct evidence for a fitness trade-off wit h amount of cover at the nest; concealed sites were less often detecte d by predators but were more risky for incubating hens. Small ptarmiga n and those in good body condition used sites with less cover, while l arge ptarmigan nested at higher elevations. Nesting success was not re lated to age or spring body condition. Responses to experience with pr edators were less clear. Successful females tended to use less cover t he following year, whereas unsuccessful females tended to change cover types, but not the amount of cover, in subsequent years. Overall, cha racteristics of nest sites showed strong seasonal patterns, suggesting that microclimate may be a more important selective pressure than pre dation. (C) 1998 The Association for the Study of Animal Behaviour.