The Breeding Ecology of Least Bitterns (Ixobrychus exilis) at Agassiz and Mingo National Wildlife Refuges

Habitat selection and reproductive effort of least bitterns (Ixobrychus exilis) were monitored in 2001 and 2002 at Mingo National Wildlife Refuge (Mingo) in southeast Missouri and 2001 through 2004 at Agassiz National Wildlife Refuge (Agassiz) in northwest Minnesota. The main objective was to study variation in the breeding ecology of least bitterns across a broad geographic area. Information gathered from nests included: nest success, egg and clutch measurements, hatchling developmental rates, and habitat measurements. Data suggest geographical variation in clutch size, relative growth patterns of hatchlings, and nest densities. A total of 74 nests were found at Mingo and 120 nests at Agassiz. Nest success was lower during incubation compared to post hatching at both sites. Data supported a trend toward larger clutches at northern locations. Tarsus and wing development of hatchlings were similar between sites, while, overall mass and culmen development was a few days less at Mingo. Nest diameter and thickness did not differ between the two sites; however, there were differences between nest height and water depth. Giant cutgrass (Zizianopsis miliacea) was the only substrate used for nest building at Mingo while Cattail (Typha spp.), sedge (Carex spp.), bulrush (Scirpus spp.) and common reed (Phragmites spp.) were all used as components for nest building at Agassiz. Birds utilized denser habitat at Agassiz. Nests in the smaller habitat patch at Mingo formed a colony while nests were more widely distributed in the larger marshes at Agassiz. Habitat conditions were variable within and among years at Mingo and Agassiz. Nevertheless, least bitterns adapted to these variable conditions because they successfully nested annually. This adaptability was apparent when bitterns nested in short residual cattail lacking overhead cover after flooding at Agassiz in 2002. Currently, there is inadequate monitoring of least bittern populations to establish the distribution and abundance in relation to habitat availability. Thus, implementation of standardized monitoring techniques would be valuable in understanding how this species utilizes marsh habitats throughout the breeding range. Standardized call response surveys lasting 5 minutes or more during the peak of nest initiation would be beneficial in gathering accurate distribution, abundance, and population trend data for the entire least bittern breeding range.