ARE RUFFED GROUSE MORE VULNERABLE TO MORTALITY DURING DISPERSAL

An increased probability of death is thought to be an inherent risk of dispersal. To examine this hypothesis, we monitored 381 radiomarked R uffed Grouse (Bonasa umbellus) over 6 yr to determine if their vulnera bility to mortality increased during dispersal. Estimates of mortality were calculated for the two distinct phases of dispersal: transience, a period of extensive movement from one area to another, and coloniza tion, the period when an animal attempts to become established in a ne w area. Among juveniles (nonresidents), approximately equal proportion s of both sexes exhibited transience, and for similar lengths of time. The difference between juvenile male and female mortality estimates w as not significant during either transience or colonization. From autu mn to spring (natal dispersal), juvenile transient dispersers did not experience greater mortality than colonization dispersers. Among adult s, transience was more common among females than males, yet no mortali ty was recorded among either sex of adult transient dispersers. Reside nts (adults) survived at a higher rate than juveniles in either transi ent or colonization dispersal. However, the cost (i.e., increased mort ality) of transience was greater for juveniles than adults, suggesting comparisons between different phases of dispersal be made within simi lar age classes. It was concluded that Ruffed Grouse, both adults and juveniles, were not more vulnerable to mortality during transient disp ersal than during colonization dispersal.