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Patterns of nocturnal flight calling over four sites in northern California and southern Oregon
The utility of nocturnal flight calls as an indicator for monitoring bird migration has been demonstrated and its usage continues to grow. However few studies have addressed how and if nocturnal flight calls can be used as a measure of migrating bird density. The objectives of this study were to 1) investigate whether nocturnal acoustic monitoring could correlate with and thus serve as a proxy to estimate the number of birds caught in mist nets during the same migratory bout, and to 2) explore the spatiotemporal variation in passerine nocturnal flight calling. Flight calls of migrating songbirds were recorded from 15 August 2008 until 10 October 2008 at four different sites. There were 4,194 warbler and/or sparrow calls (12 species) detected and classified into individual species or species complex groups. In addition, more than 12,000 Swainson’s thrush (Catharus ustulatus) calls were recorded with 3,420 occurring in a single night. Banding stations operated concurrently with acoustic stations, and this enabled direct data comparison. In some cases model analysis showed the number of flight calls detected may have an impact on the number of birds caught in mist nets. Additional methods such as radar technology and census counting usable for comparison could help tease apart the complex interaction between flight call density and the number of birds in a given area.