Recruitment of juveniles is an important vital rate that influences population growth and is fundamental to understanding trends in population size. Estimates of recruitment are often focused on the period just after hatching (prefledgling stage), which is typically the lowest survival period and often the most variable. Few studies examine true recruitment—survival from hatching to entering the breeding population—although this information is more relevant to understanding population trends. We studied the recruitment of Gunnison Sage-Grouse (Centrocercus minimus), a federally threatened species in the U.S., to examine the relative importance of chick and juvenile survival to recruitment patterns. We evaluated recruitment from 2005 to 2010 by combining separate estimates of chick survival (hatching to 30 days of age) and juvenile survival (31 days of age to the start of the first breeding season). To explain variation in these survival rates, we examined the effects of population, individual (i.e. age), and temporal (within-year and among-year differences) factors associated with recruitment of Gunnison Sage-Grouse. The factors that most explained juvenile survival rates were temporal (among-year trends and within-year seasonal effects). Chick survival rates varied by population, and daily chick survival increased with chick age. We found a slight negative trend in chick survival and a strong negative trend in juvenile survival from 2005 to 2010. The overall recruitment rate declined from 0.32 (± 0.09 SE) in 2005 to 0.04 (± 0.03 SE) in 2010. This decline coincided with a decline observed in population index data, which was not reflected in other demographic data. If survival had not been monitored past 30 days of age, estimates of recruitment would have remained relatively stable. This work highlights the importance of monitoring juvenile survival, as it may influence population dynamics.
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Vol. 118 • No. 3