Survival and Growth of Northern Bobwhite Chicks in Western Oklahoma
Lusk, Jeffrey J., Guthery, Fred S., Cox, Scott A., Demaso, Stephen J., Peoples, Alan D., The American Midland Naturalist
Knowledge of chick ecology is essential for understanding and managing populations of northern bobwhites (Colinus virginianus). We report on chick survival and growth in western Oklahoma during 1997-2002. We used Cox proportional hazard models to estimate chick survival rate. Covariates included mass at time of capture, Julian day of capture and year. Chick mass was the only significant covariate in the model. Survival depended on time since capture because survival increased with chick mass, which concomitantly increased with time since hatch. Because of the dependence of survival on chick mass, we modeled chick growth as a function of age using a logistic model (r^sup 2^ = 0.98). Growth-rate was highest at day 35 post-hatch, which was around the mean capture-to-death survival time for chicks (30.0 ± 4.4 d SE). Our results suggest that the 30-35-d period post-hatching is a critical period in chicks' lives.
The number of bobwhites in the autumn population is the sum of the number of adults and juveniles surviving through the breeding season. To date, adult survival has received the most research attention, and the survival rate of chicks is among the least investigated aspects of bobwhite ecology (Roseberry and Klimstra, 1984; Guthery, 2002). Knowledge of the demography of bobwhite chicks during the first few weeks of life is essential for understanding fluctuations in autumn populations.
For short-lived species such as the bobwhite, population growth rate is most sensitive to changes in fecundity (Lebreton and Clobert, 1991) and, in as much as it is related to fecundity, recruitment. For bobwhites, 66% of the variation in recruitment was explained by the number of chicks hatched per hen (fecundity) (Klimstra and Roseberry, 1975). Increasing juvenile mortality by 45% (from 15 to 60% mortality) had a 2.5× greater impact on recruitment than a 60% increase (from 20 to 80% mortality) in adult mortality (Roseberry, 1974). Roseberry and Klimstra (1984) suggested that, because of this relationship between fecundity and recruitment, juvenile survival might play a secondary role in determining autumn population size. However, the effect of juvenile survival on recruitment and autumn population size is still considerable; Roseberry (1974) reported that fecundity and juvenile survival were equally important to recruitment.
Further, such information will provide wildlife managers with a better understanding of the ecology of the bobwhite. Our objectives were to model chick survival rates as a function of mass at capture, Julian day of capture and year. We used radiotelemetry to track individually marked chicks during 1997-2002. This allowed us to follow survival of chicks past the brood-rearing stage. We modeled survival using a Cox proportional hazards model. Given our best survival model, we then modeled the growth rate of bobwhite chicks as a function of time since hatch to better understand the relationship between mass and survival rate through time. We also derived an estimator of hatch-to-November survival of chicks that was independent of telemetry data; this was done to check telemetry-based survival estimates, which are usually biased low for bobwhites (Guthery and Lusk, 2004).
Study area.-This study was part of a long-term ( 1991-2002) research program conducted on the Packsaddle Wildlife Management Area in southern Ellis County, 40 km north of Cheyenne, Oklahoma. The Packsaddle encompassed 6475 ha of mixed-grass prairie. DeMaso et al. (1997; and references therein) provided a detailed description of the Packsaddle.
Capture and telemetry.-This study was initiated in 1997 as an extension of the work done by DeMaso et al. (1997). However, rather than estimate chick survival by following radiomarked adults, we monitored radiomarked chicks continuously during 1997-2002 until death, disappearance or entrance into the adult population. Nests, found by following radiomarked adults, were monitored to determine hatch date, and chicks were caught 1-5 d after hatching. …