Date of Award

5-1-1998

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

Abstract

American bittern breeding populations have been declining for at least 3 decades. Lack of information on life history traits precludes the conclusive determination of causal factors. The objectives of this study, conducted in northwest Minnesota, were to 1) estimate summer home-range size of American bitterns, 2) document local movements and habitat use by American bitterns, and 3) refine capture and marking techniques. Results of this study should facilitate further research and may be useful to wildlife managers for implementing management practices to benefit bitterns.

Radio-marked American bitterns were systematically relocated from May through August, 1996-97. Seasonal core-areas and low-use areas were determined from 18 males and 2 females. Male breeding home-ranges averaged 210 ha (n = 22) and did not differ (P > 0.05) from average post-breeding home-range size (152 ha, n = 16). Distances of male post-breeding dispersals ranged from 0.2 - 20.0 km; however, 64% were under 2.0 km (n=22).

Proportion of habitat types within 5 male core-areas did not differ (P > 0.05) from that in low-use areas except for cattail, which was less abundant in core-areas (P < 0.05). Habitat use was disproportional to abundance (P < 0.005). Cattail, deciduous trees, and willow/grassland were avoided while open water was preferred (P < 0.10). Small sample size (n = 5) likely produced spurious habitat use results.

No refinements were made to marking techniques. I; is recommended that the tape recording used with the mirror trap be modified. Improving nest search techniques might aid in the development of effective capture methods for females. Fidelity to breeding and post-breeding home-ranges was 57% and 80%. respectively. Accuracy of predicting sex based on oill and tarsus length was 93%. Upland nests produced more fledglings per nest than wetland nests. The restoration and preservation of shallow wetlands and tall grasslands are recommended as management practices to benefit American bitterns. Additional research at Agassiz NWR, as well as other physiographic regions and wintering grounds, is needed to conclusively identify factors responsible for bittern population declines.

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