In this talk we present a Bayesian nonparametric model for count data in an animal population with size not assumed to be constant since individuals can arrive and depart from the site during the sampling period. A nonparametric prior known as Polya Tree is
used for modelling the bivariate density of arrival and departure times. We apply the model to a population of Great Crested Newts collected in a local site from the Durrell Institute of Conservation and Ecology.