Annual Survival of Snail Kites in Florida: Radio Telemetry versus Capture-Resighting Data

-We estimated annual survival of Snail Kites (Rostrhamus sociabilis) in Florida using the Kaplan-Meier estimator with data from 271 radio-tagged birds over a three-year period and capture-recapture (resighting) models with data from 1,319 banded birds over a six-year period. We tested the hypothesis that survival differed among three age classes using both data sources. We tested additional hypotheses about spatial and temporal variation using a combination of data from radio telemetry and singleand multistrata capturerecapture models. Results from these data sets were similar in their indications of the sources of variation in survival, but they differed in some parameter estimates. Both data sources indicated that survival was higher for adults than for juveniles, but they did not support delineation of a subadult age class. Our data also indicated that survival differed among years and regions for juveniles but not for adults. Estimates of juvenile survival using radio telemetry data were higher than estimates using capture-recapture models for two of three years (1992 and 1993). Ancillary evidence based on censored birds indicated that some mortality of radio-tagged juveniles went undetected during those years, resulting in biased estimates. Thus, we have greater confidence in our estimates of juvenile survival using capturerecapture models. Precision of estimates reflected the number of parameters estimated and was surprisingly similar between radio telemetry and single-stratum capture-recapture models, given the substantial differences in sample sizes. Not having to estimate resighting probability likely offsets, to some degree, the smaller sample sizes from our radio telemetry data. Precision of capture-recapture models was lower using multistrata models where region-specific parameters were estimated than using single-stratum models, where spatial variation in parameters was not taken into account. Received 19 December 1997, accepted 21 September 1998. FOR MANY LONG-LIVED avian species, population persistence is more sensitive to annual survival than to fecundity (Mertz 1971, Nichols et al. 1980, Beisssinger 1995). Despite this, reliable estimates of survival are unavailable for many species, although extensive effort often is expended in estimating reproductive parameters. Investigators also must choose among available techniques for estimating demographic parameters. This selection often is based on logistic constraints, or unfamiliarity with potential estimators, rather than how procedure selection might influence resulting parameter estimates. Given current threats to 5 Present address: Station Biologique de la Tour du Valat, Le Sambuc, 13200 Arles, France. E-mail: [email protected] many populations, reliable demographic data are essential for effective conservation arguments in the context of alternative management scenarios. The Snail Kite (Rostrhamus sociabilis) is an endangered raptor whose range in the United States is limited to central and southern Florida (Sykes et al. 1995). Florida's wetlands have been severely altered during the past century by drainage, impoundment, changes in water flow regimes, increased nutrient loading, and invasion by exotic plants and animals (Walters et al. 1992, Davis and Ogden 1994). These disturbances have resulted in one of the largest ecosystem restoration projects ever undertaken (Davis and Ogden 1994). The Snail Kite, like many other species, is potentially influenced by these and other changes (Bennetts et al. 1994).