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Methods for Dickcissel night flight call monitoring in south Texas
Each recording station consists of a single pressure zone microphone (PZM) with a directional pick-up pattern (Evans and Mellinger 1999), a cassette deck that functioned as a signal amplifier (Optimus SCT-86 Radio Shack), a frequency equalizer to boost mid range frequencies and reduce high and low frequencies (Optimus Radio Shack), and a computer system (at least a 500 mHz PC). The microphones are mounted on the roof of a building at each location and aimed at the zenith. They are positioned in locations that minimize pickup of terrestrial environmental noise (e.g., insects and auto traffic). The broad flat roofs of typical high schools are well-suited for this purpose. Field tests with this microphone during the day have revealed that in conditions of low ambient noise it can detect flight calls of Dickcissels out to a range of at least 500 meters. The microphone pick-up pattern is largely determined by its housing which allows essentially unimpeded pickup of sound in a 90 degree cone expanding upwards into the sky. Call notes emanating from areas outside this cone can also be picked up by the microphone due to the spherically expanding nature of sound waves, but such calls are subject to reduction in sound levels because they are impeded by the external microphone structure before reaching the microphone element. Structures in the vicinity of the microphone (e.g., trees, walls, etc.) in no case impede sound in the field of the 90 degree cone of sensitivity. Such structures do vary at each location in impeding sound that emanates from outside this 90 degree cone.
Sound received by the microphone is relayed by audio cable to similar IBM compatible computers operating inside the buildings at each location. Standard 16 bit soundcards are used to digitize the received sound and software developed by Old Bird, Inc. is used to analyze the audio stream and detect sounds with characteristics of the Dickcissel flight call. Detections are verified as Dickcissel calls by visual verification of spectrograms and by listening to soundfiles.
The volume of sky that a recording station detects Dickcissel calls varies due to differences in the terrestrial environmental noise at each location. To assess this variable, first, each station's inherent amplification is calculated. A recording of a Dickcissel call is played back at a constant sound level and fixed distance to each acoustic field station. Ambient environmental noise is then measured every second and averaged over each hour of recording. This information and the variable station amplification levels are factored in to determine a relative level of ambient noise at each station. The chief component of terrestrial background noise is song from several species of crickets. Because most of the stations are located on large flat roofs away from vegetation, there is little difference in ambient noise between most stations.
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