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Powell, A.N. 1991. Monitoring black tern populations in Minnesota in association with the USFWS waterfowl production survey. Report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 24 pp.


In 1990, attempts were made by the U.S. Fish and Wildlife Service (USFWS) to incorporate several nongame species, including black terns (Chlidonias niger), into their annual waterfowl breeding pair count. The report that was generated from this initial attempt was reviewed by USFWS and Minnesota DNR personnel, and several problems with using the waterfowl count to census nongame wetland birds were identified. Two of the major concerns with the waterfowl count were the phenology of black tern breeding biology in Minnesota, and standardization of census methodology. The 1991 study was conducted to evaluate the use of the USFWS waterfowl breeding pair counts as a viable index for black tern populations in Minnesota. The objectives of this study were to (1) address the interpretation of black tern observations made during the count periods by the USFWS and myself, (2) indicate whether surveys should be conducted at alternate times during the breeding season to make the black tern count meaningful, and (3) standardize and/or improve censusing methodology.

Four-square-mile study plots selected by the USFWS were surveyed in Polk and Kandiyohi counties, where black terns were counted in 1990. The first waterfowl count was conducted on 14 and 15 May, and the second count was conducted on 21 - 22, and 30 May. I returned to the study areas on two more occasions during the breeding season to determine breeding activity, colony locations, and tern populations. Census results from this study were compared to results from 1990.

Black terns were not seen on USFWS plots in Polk and Kandiyohi counties during the first census, but were observed flying over some of the ponds in the study plots during the second census. These individuals were not engaged in courtship or nesting activities. No black terns nested on any of the ponds included in the waterfowl census in Polk, Kandiyohi, Kittson, or Roseau counties in 1991. Two colonies of black terns were found in locations within two miles of the USFWS study plots in Polk County, and black terns nested in a wetland several miles from a study plot in Kandiyohi County. Black terns in these colonies were incubating eggs in mid-June, and chicks fledged by mid-July. Colonies were located in semi-permanent wetlands (Class IV), with cover type 2 (open water covering 5-95%, with dense patches or diffuse open stands of emergent cover). Terns in both areas nested in shallow wetlands characterized with patches of sedges.

Problems associated with the USFWS waterfowl count for censusing black terns included sampling design and timing of the survey. Plots for the waterfowl counts were selected for estimating duck populations in the prairie pothole region of Minnesota, and miss many actual and potential black tern nesting sites. In addition, the timing of the waterfowl count is too early to determine whether black terns are nesting on selected study plots. Although black terns were observed during the second count in late-May and early-June, they were foraging, and not displaying any courtship behaviors. Because the terns didn't remain on the wetlands within study plots in Polk and Kandiyohi counties, I advised against using these results to make management recommendations, habitat evaluations, or population estimates for black terns based on the USFWS waterfowl breeding pair counts. A census in mid- to late-June would be more appropriate for monitoring breeding black terns.

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