The use of resighting data to estimate the rate of population growth of the snail kite in Florida

The rate of population growth (λ) is an important demographic parameter used to assess the viability of a population and to develop management and conservation agendas. We examined the use of resighting data to estimate λ for the snail kite population in Florida from 1997-2000. The analyses consisted of (1) a robust design approach that derives an estimate of λ from estimates of population size and (2) the Pradel (1996) temporal symmetry (TSM) approach that directly estimates λ using an open-population capture-recapture model. Besides resighting data, both approaches required information on the number of unmarked individuals that were sighted during the sampling periods. The point estimates of λ differed between the robust design and TSM approaches, but the 95% confidence intervals overlapped substantially. We believe the differences may be the result of sparse data and do not indicate the inappropriateness of either modelling technique. We focused on the results of the robust design because this approach provided estimates for all study years. Variation among these estimates was smaller than levels of variation among ad hoc estimates based on previously reported index statistics. We recommend that λ of snail kites be estimated using capture-resighting methods rather than ad hoc counts.