Clutch Size Variation and Patterns of Attrition in Fecundity of Willow Ptarmigan

Abstract

Using a compartmental life history model we comapred clutch sizes and patterns of attrition in fecundity from egg to reproductively mature offspring for both first nests and replacement clutches in two populations of Willow Ptarmigan (Lagopus lagopus) at La Perouse Bay (LPB) and Chilkat Pass (CP) in northern Canada. From her initial clutch of 10.8 eggs, a female at LPB fledged on average 2.9 offspring, a reproductive efficiency of 27%. Of hens losing first clutches, 83% renested and these subsequent clutches contributed 23% to annual fecundity. A female at CP laid an average of 8.1 eggs in the first clutch and fledged 3.0 (37%) offspring. About 67% of females renested if they lost first nests, and renests contributed 14% to net annual fecundity at CP. Over 4 yr at LPB, clutch size, patterns of attrition in fecundity, and return rates of females did not vary, but at CP all measures of fecundity, total nest failure, and female return varied dramatically over 7 yr. Despite the larger clutch size of LPB hens, the overall mean values of net annual fecundity and the rate of return (a measure of survival) of females between years did not differ between the two popualtions. However, patterns of attrition in fecundity differed greatly between sites. Differences in clutch size and renesting ability between LPB and CP may be caused by differences in resource availability or differences in predictability of clutch predation. Clutch size and renesting rate may be higher at LPB to cope with high and predictable egg loss. Clutch loss at CP was variable and unpredictable, and the lower clutch size and renesting rate there may have developed because of lower food availability. We suggest ways to test these hypotheses.