Factors influencing age-related reproductive success in the willow ptarmigan

-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. Received 8 August 1983, accepted 28 March 1984. HYPOTHESES concerned with the factors that influence the onset of breeding in birds focus at three different levels: (1) on general environmental cues, such as changes in photoperiod or rainfall, that explain differences in timing among species or among geographical areas (reviewed by Wingfield 1983); (2) on specific extrinsic factors, such as weather or the availability and quality of food, that explain differences in timing among populations or among years (e.g. Perrins 1970, Zwickel 1975, Greenlaw 1978); and (3) on intrinsic factors, such as age and experience of the female or the male, or on genetic factors that explain differences in timing among individuals within populations (e.g. Perrins and Moss 1974, Finney and Cooke 1978, Harvey et al. 1979, van Noordwijk et al. 1980). In this paper we concentrate on intrinsic factors, specifically the influence of age on reproductive performance. Age and experience have long been considered to be important variables in explaining not only the timing of breeding but the reproductive output as well. In many species, older birds lay earlier and have higher reproductive success than first-time breeders (e.g. Nelson 1966, Crawford 1977, Blus and Keahey 1978, Middleton 1979). This has been attributed to ' Present address: Department of Zoology, University of Alberta, Edmonton, Alberta T6G 2E9, Canada. the inexperience of young birds in locating food required for egg formation (Lack 1968) and to social inhibition of young birds when they compete with adults for breeding sites (WynneEdwards 1962). In some species, however, firsttime breeders do just as well as older birds (e.g. Leinonen 1973, Myrberget 1967). In this paper, we compare timing of breeding, reproductive output, and territory size of adult and yearling Willow Ptarmigan (Lagopus lagopus alexandrae) of both sexes. We then compare Willow Ptarmigan with other species of grouse and discuss the effects of mating-system type, population demography, and other factors on the timing of breeding and reproductive output of young birds.