Individual study: Broadcasting heron alarm-distress calls is mostly ineffective for frightening black-crowned night herons Nycticorax nycticorax and great blue herons Ardea herodias from Watson Lake Fish Rearing Unit, Colorado, USA
Andelt W.F. & Hopper S.N. (1996) Effectiveness of alarm-distress calls for frightening herons from a fish rearing facility. The Progressive Fish-Culturist, 58, 258-262
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Scare birds from fish farms
A before-and-study in May-June 1993 at a trout-rearing farm in Colorado, USA (Andelt & Hopper 1996), found a 48% reduction in black-crowned night heron Nycticorax nycticorax numbers following the broadcasting heron alarm/distress calls for 11 days (pre-treatment average 77 birds; treatment 40; post-treatment 69). However, numbers and the proportion remaining increased from nights over the treatment phase, indicating habituation. Great blue heron Ardea herodias numbers were unaffected (pre-treatment 15; treatment 13; post-treatment 16. Herons were counted during six pre-treatment (12-13 to 18-19 May 1993), five treatment (21-22 May to 31 May-1 June 1993), and five post-treatment (1-2 to 11-12 June 1993) nights. Calls were broadcast through each night of the 11 day treatment period: 15-sec sequences of night heron calls followed by 14 min without calls, then a similar sequence of great blue heron calls.