SPRING POPULATION ESTIMATES OF ROCKY MOUNTAIN GREATER SANDHILL CRANES IN COLORADO

Authors

Douglas S. Benning, U.S. Fish and Wildlife Service
Roderick C. Drewien, University of Idaho
Douglas H. Johnson, USGS Northern Prairie Science CenterFollow
Wendy M. Brown, University of Idaho
Erwin L. Boeker

Date of this Version

1997

Document Type

Article

Citation

Benning, Douglas S., Roderick C. Drewien, Douglas H. Johnson, Wendy M. Brown, and Erwin L. Boeker. Spring population estimates of Rocky Mountain greater sandhill cranes in Colorado. In: Urbanek RP, Stahlecker DW, eds. 1997. Proceedings of the Seventh North American Crane Workshop, 1996 Jan 10-13, Biloxi, Mississippi. Grand Island, NE: North American Crane Working Group. pp. 165-72.

Comments

Used by permission of the North American Crane Working Group.

Abstract

We developed an aerial survey to estimate the Rocky Mountain Population (RMP) of greater sandhill cranes (Grus canadensis tabidn) (greaters) annually during March 1984-95 in the San Luis Valley, Colorado (SLV). We flew aerial transects covering a 21,220-ha contiguous area. Aerial photographs of crane flocks were taken to correct for estimation errors. We employed ground surveys to correct for presence of lesser sandhill cranes (G. c. canadensis) (lessers), which mixed with RMP cranes. To adjust for RMP cranes not in the survey area, we made ground counts in traditional use areas outside the SLY. During 1984-95, 5 comparable aerial surveys and 1 ground count yielded estimates of the RMP ranging from 16,220 to 21,496 (X = 19,223). Counts indicated that the popUlation was stable. Cloud cover, partial snow cover, and lack of background color contrast in certain habitat types reduced visibility of cranes. Increases in non-RMP cranes mixed with RMP flocks are jeopardizing the accuracy of counts and assessment of population trend. An alternative census method may be needed.