Testing the effectiveness of a novel approach to measure a large roosting congregation in a wetland ecosystem

The National Wetland Trust constructed a 1400 m long pest exclusion fence around a 11 hectare site at Rotopiko and all mammals except mice have been eradicated from inside the fenced area. Since the completion of the pest proof fence, the number of roosting birds has increased dramatically. By removing mammalian pests, an unexpected sanctuary has been created for communal roosting birds such as starlings (Sturnus vulgaris) and sparrows (Passer domesticus). There is growing concern about the large amounts of bird faeces produced each day and the associated nutrient load particularly as the pest fence encircles a low nutrient peat lake. In this study two methods were developed for estimating the relative abundance of the large roosting congregation, based on (1) gridded plates for gathering guano and (2) the acoustic energy of the roost cacophony. The results showed that these methods were suitable for measuring the characteristics of the roosting community at multiple scales. For example, large variance in the abundance of roosting and diurnal birds, and minor changes in the roosting community according to seasonal fluctuations. The methods could support the management of the issue through being applied to assess and quantify the relative efficacy of preventive or control methods deployed to reduce the number of exotic birds. The findings of this study are site specific; however, the guano plates and sound recorders could be implemented to estimate large bird numbers at other sites facing a roosting bird problem.