Timing of Reproduction in Two Populations of Willow Ptarmigan in Northern Canada

Abstract

We examined extrinsic and intrinsic factors that might influence the timing of clutch initiation and determined the relationship between timing and reproductive success in two populations of Willow Ptarmigan (Lagopus lagopus) in northern Canada. The mean date of clutch initiation varied up to 16 days over 8 yr and appeared to be related to the timing of snow melt. In one population females laid more synchronously when the onset of breeding was late, but in the other they did not. Laying was not correlated with population density or with proportion of yearling hens in the population. Older hens paired with adult males initiated nesting earlier than yearlings or adult hens paired with yearling males. Females that switched mates from one year to the next laid at the same time as those paired with their previous mates. The timing of clutch initiation or hatch was consistent for individual females from one year to the next in one population, but not in the other. Body size, estimated by wing length, was not correlated with time of laying, but hens with larger body mass laid earlier. Females that nested early laid larger clutches than those laying late. In only 1 yr of 6, a year with very high nest predation, late-nesting hens lost a lower proportion of their clutches. Early-nesting hens hatched more eggs from successful nests than late-nesting hens, but the number of young that fledged and subsequently returned to the population was similar for hens nesting in early, middle, and late reproductive periods. We suggest that hens capable of laying early do so to renest if their clutches are destroyed by predators.