Patterns of Avian Migration in California: An Analysis and Comparison of Results from NEXRAD Doppler Weather Radar and Multiple Mist-net Stations

摘要

Studying the spatiotemporal patterns of migrants in flight is challenging given that waterfowl, shorebirds, and most passerines migrate exclusively at night (Berthold 1993). An early method used to quantify nocturnal migration patterns was to count the number of birds silhouetted against disk of the full moon or by observing birds in flight from an aircraft (Nisbet 1959, Lowery 1966, Bellrose 1971). However, the development of automated technologies, such as radar, acoustic sensing, and infrared technology has largely opened this avenue of research (Millikin 2005). Each of these automated technologies has its strengths and limitations, depending on the particular research objective. For example, short-range radar can provide information on local 'traffic rates', which can be used for conducting site impact assessments (e.g. the potential impact of towers and wind turbines). Acoustic technology, on the other hand, can also provide species specific data. However, both short-range radar and acoustic sensing technology are applicable at only a relatively local scale (Millikin 2005). However, NEXRAD ('Next Generation Radar') Doppler weather radar (hereafter referred to as WSR-88D, for Weather Surveillance Radar 88 Doppler), has been shown to be an effective tool for studying nocturnal spatiotemporal migration patterns across a broad geographic range (Gauthreaux 1992, Gauthreaux 1998, Gauthreaux 2003b, and Diehl 2005). WSR-88D can provide data on relative migration density, velocity, and direction of flight for flying vertebrates (birds and bats) within a radius of up 240 km radius around the radar station (Gauthreaux 2003a). Under the appropriate conditions, WSR-88D can potentially indicate areas where migration is concentrated, which may help identify important stop-over or roosting sites (Russell 1998, Diehl 2005).