Breeding and Age in Female Black Grouse Tetrao tetrix

Abstract

This study evaluates breeding success of 78 (25 yearling and 53 adult) Black Grouse females in central Sweden. Black Grouse females were captured and radiomarked in late winter or early spring. Females were monitored from egglaying until late July/ early August when broods were flushed. Yearlings were significantly smaller (body weight and wing length) than adult females captured at leks. All yearlings made breeding attempts. Yearlings laid significantly smaller first clutches and eggs than adult females. Nest predation was similar for both age classes but yearlings had a lower tendency to renest. More yearlings than adult females lost their entire brood during the 5 weeks after hatching, 82% vs 35%, respectively. Only 2 of 25 (8%) yearlings raised chicks to 5 weeks of age compared with 20 of 50 adult females (40%). Adult birds raised on average 1.72 chicks per female compared with 0.16 for yearlings. Mechanisms that can explain the age-specific breeding success, such as maternal condition and territorial behaviour are discussed. It is suggested that the large difference in age-specific breeding success observed in this study was a result of the overall habitat structure in the area.