Thermal circulation affects Black Vulture Coragyps atratus soaring behaviour in the vicinity of two airports in south-east Brazil.

Abstract

The growth of air traffic all over the world faces an increasing risk of aircraft collisions with birds. The Black Vulture (Coragyps atratus), being a large soaring species living across the Americas, presents a serious hazard to airplanes. The reduction of this threat without harm to birds is a complex task requiring thorough investigations of their behaviour. Various natural and anthropogenic factors can affect the distribution of soaring Black Vultures in the surrounds of airports. Here we focus on the impact of meteorological parameters (i.e. air temperature, relative humidity, wind speed and atmospheric pressure) on Black Vulture flight at two study sites. In the course of one year, from September 2012 to August 2013, we surveyed with binoculars the soaring activity of Black Vultures around Amarais and Presidente Prudente airports in the southeast of Brazil. The study areas were characterized by tropical climatic conditions with mild dry winter and hot rainy summer. We found that the frequency of soaring flights depended on the wind speed, a proxy of the strength of thermals (upward air flows). This finding is consistent with habits of Black Vultures using thermals for soaring. In contrast, air temperature, air humidity and atmospheric pressure did not affect their activity. We also showed that the different seasons affected the birds’ behaviour, but the degree of their influence depended on the level of anthropogenic pressure in the soaring terrain. Two contrasting types of daily soaring activity appeared: a plateau-like trend and a tendency of two peaks at morning and afternoon with a pronounced drop between them. Our findings can be used to inform methods to reduce the risk of collisions between aircraft and Black Vultures at our study sites, and our analytical approach could be applied elsewhere.