Distance sampling as an effective method for monitoring feral pigeon (Columba livia f. domestica) urban populations

SUMMARY Current methods for estimating feral pigeon (Columba livia f. domestica) population size and for monitoring population trends are mainly based on indices, which according to the current literature on wildlife census methods often produce biased results. Distance Sampling techniques have never been used in this context, even though they could theoretically produce absolute abundance estimates at relatively low costs. The aim of this paper was to investigate the performance of Distance Sampling to census feral pigeons, and to compare these results with those obtained by using Quadrate Counts, a widespread method for monitoring these birds. Surveys were performed in Pisa (Italy) in two different periods of the year 2004 (end of January–beginning of February, and November), which correspond to minimum (January–February) and maximum (November) numbers for pigeon populations. We considered 40 line transects each about 250 m long for Distance Sampling, and 40 250×250 m cells for Quadrate Counts. In both cases, sampling units were randomized in a stratified design. In contrast to Quadrate Counts, Distance Sampling detected the predicted increase of abundance from January–February to November with an acceptable precision and no increase of costs per survey. Even though the possible biases (due to the not rigorously random distribution of transects and to the spiked nature of collected distance data) should be further investigated, results suggest that Distance Sampling is a viable and efficient alternative to the traditional methods used to estimate feral pigeons population size and to monitor trends