The effectiveness of call-response surveys for detecting least bitterns

We determined the effectiveness of broadcasting recordings of conspecific vocalizations (call-response surveys) for monitoring least bittern (Ixobrychus exilis) populations within an 8,000-ha wetland complex in western New York, USA, during 2000. Call-response trials on 15 radiomarked males increased least bittern calling (25.4% of trials) compared to passive listening (9.5%; P= 0.010), and response to broadcasts was higher near nest initiation (46.9% of trials) versus incubation (11. 1 %) and hatching (11. 1 %; P < 0.005). Call-response surveys are all effective monitoring tool for least bitterns, but surveys should be conducted when most birds are initiating nests. Response rates also can be used to estimate true abundance of male least bitterns in a marsh.