Data from: Estimating transient populations of unmarked individuals at a migratory stopover site using generalized N-mixture models

1. Migration counts are popular indices used to monitor population trends
over time. Advanced analytical methods for estimating abundance of
unmarked, open populations now incorporate population growth models and
simultaneously test for covariate effects on abundance and detection
probability. However, estimating population abundance at a staging site is
complicated by daily immigration and emigration of unmarked individuals.
2. We applied a set of generalized N-mixture models to simulated count
data to test their applicability for transient populations. Using
simulated datasets, parameters were unbiased when the apparent survival
rate varied within a season or was mis-specified in a model, but not when
the immigration or detection probability was mis-specified. 3. With
knowledge from the simulated data, we applied these models to daily counts
of staging migratory shorebirds and estimated daily abundances accounting
for variation in the detection and immigration rates. Daily counts of
ruddy turnstones (Arenaria interpres) staging at Westhampton Island, New
York, were collected during northward migration (1997–1999). We tested the
effects of weather and tides on detection probability, and we modeled
within-season variation in immigration rates as a function of time. 4.
Covariates affecting the detection probability differed among years, but
tide height consistently was correlated with detection probability.
Accounting for detection and immigration rates, the predicted maximum
single-day populations of ruddy turnstones were 172%, 165%, and 129% of
the observed counts for each year. 5. Synthesis and applications.
Management and conservation plans for migratory species require abundance
estimates that are near the true population size though they are difficult
to obtain. Our study is the first empirical application of the generalized
N-mixture model that incorporates temporal trends in immigration and
estimates daily abundance of a staging unmarked migratory population.
Correct estimation of population sizes and the environmental factors
affecting them can aid the conservation prioritization of species and
staging sites. Moreover, the use of generalized N-mixture models can
improve our understanding of the environmental factors that shape
migratory movements.