Factors influencing counts in an annual survey of Snail Kites in Florida

Snail Kites (Rostrhamus sociabilis) in Florida were monitored between 1969 and 1994 using a quasi-systematic annual survey. We analyzed data from the annual Snail Kite survey using a generalized linear model where counts were regarded as overdispersed Poisson random variables. This approach allowed us to investigate covariates that might have obscured temporal patterns of population change or induced spurious patterns in count data by influencing detection rates. We selected a model that distinguished effects related to these covariates from other temporal effects, allowing us to identify patt erns of population change in count data. Snail Kite counts were influenced by observed differences, site effects, effort, and water levels. Because th ere was no temporal overlap of the primary observers who collected count da ta, patterns of change could be estimated within time intervals covered by an observed, but not for the intervals among observers. Modeled population change was quite different from the change in counts, suggesting that analy ses based on unadjusted counts do not accurately model Snail Kite populatio n change. Results from this analysis were consistent with previous reports of an association between water levels and counts, although further work is needed to determine whether water levels affect actual population size as well as detection rates of Snail Kites. Although the effects of variation i n detection rates can sometimes be mitigated by including controls for fact ors related to detection rates, it is often difficult to distinguish factor s wholly related to detection rates from factors related to population size . For factors related to both, count survey data cannot be adequately analy zed without explicit estimation of detection rates, using procedures such a s capture-recapture.