Date of this Version
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.
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.