MIGRATING BIRDS STOP OVER LONGER THAN USUALLY THOUGHT: REPLY
Résumé
The seasonal long-distance movements of migratory birds are punctuated by periods of rest and refueling termed stopovers. The duration of these periods is an important variable in the biology of migratory species. Capture-recapture provides data for the estimation of stopover duration in the many species for which individual tracking is not feasible. Schaub et al. (2001) proposed a new method for the analysis of such data. This note suggests that the method leads to erroneous estimates of mean stopover duration. Estimation of stopover duration in the absence of mortality is mathematically identical to estimation of life expectancy in the absence of emigration. Life expectancy at birth is defined as L p(a) da (1) 0 0 where survivorship p(a) is the probability of surviving at least to age a (a is continuous and p(0) 1) (Keyfitz 1985, Lotka 1998). Relating this to stopover duration, we interpret ''age'' as time since arrival, and the sur-vivorship curve p(a) as the proportion of birds remaining in the population a time units after arriving. The Cormack-Jolly-Seber (CJS) open population model (Cormack 1964, Seber 1982) provides estimates of survivorship that underlie both the analysis of Schaub et al. (2001) and this note. The stopover population is subject to recruitment and loss between samples. Stopover durations are assumed to be short, so that mortality may be ignored, and the rate of departure is given by 1 t , where t is the time-specific CJS ''survival'' parameter (probability that an animal in the population at time t will also be in the population at time t 1). Because ''survival'' is time dependent under this model, the cohort of birds arriving at time i experiences a particular ''survivorship'' curve that may be designated p i (a). In the stopover case that we Manuscript