Factors Influencing Age-Related Reproductive Success in the Willow Ptarmigan

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Abstract
In many species, adult birds lay earlier and have higher reproductive success than do yearlings. We found no difference, however, between adult and yearling female Willow Ptarmigan (Lagopus lagopus alexandrae) in date of clutch initiation, clutch size, hatching success, number of fledged young, or territory size. Adult females defended their broods more vigorously, and more were observed with broods, a situation suggesting that more yearlings lost their eggs or young and did not renest. Pairs composed of two adults produced more fledged young than did pairs composed of two yearlings, but clutch size and date of clutch initiation were similar in the two groups. We suggest that the following factors may allow yearlings to reproduce almost as successfully as adults: (1) a reduction in competition with adults for territories because of high population turnover, (2) the presence of extensive male parental care and precocial young, and (3) a dependence on a food source that is readily available and can be obtained without specialized foraging skills.