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Spring 2000 relative ambient noise levels

The table above shows the relative ambient noise level in the 4.0-5.5 kHz  frequency range at the nine acoustic recording stations in operation in south Texas in Spring 2000.  Stations with a zero reading have roughly the same level of background noise.  All other stations had relatively higher levels of background noise.  6 dB is a doubling of the level of background noise so Rio Grande City had a background noise twice as loud as Roma High School.  Rio Grande City High School had air conditioners on the roof of the school that operated through the evening and were the cause of the increased noise levels.  The chief variable in background noise at other stations was the proximity to singing insects.  Equipment problems at Laguna Atascosa did not allow consistent monitoring of the background noise at that station.  That station had a relatively large amount of insect noise and the ambient noise levels were likely quite high; possibly similar to the 10 dB at the South Padre Island Station that also had vegetation with singing insects in close proximity.

The documenting of Dickcissel calls in spring 2000 monitored even weak Dickcissel calls and therefore changes in the background noise between stations would potentially have a significant affect on the number of Dickcissel calls detected.  A likely example of this bias is evident when comparing the night to night data from Roma High School and Rio Grande City High School.  These two stations were at the far western end of the monitoring transect and only 20 km (~ 12 miles) apart.  The density gradient of the Dickcissel flight indicated by the transect tended to decline from McAllen High School westward.  But due to the air conditioners on the roof Rio Grande City High School, the background noise averaged twice as loud (6dB) as Roma High School.  This may have been the reason why on many nights Roma High School registered more Dickcissel flight call detections than Rio Grande City High School  (see graph below).