Estimating Densities of Breeding Canvasbacks and Redheads

Methods of estimating breeding densities of canvasbacks (Aythya valisineria) and redheads (A. americana) were evaluated by comparing variances of estimates arising from ground counts on 12 259-ha sections. Variances were similar among indexes used to indicate breeding pairs. Mainly by reason of past use and simplicity, the pair-plus-lone-male index is recommended for canvasbacks, and the total-female index for redheads. For isolated blocks, precision of density estimates improved with increasing pair densities, up to about 2.3 indicated pairs/km2. When sections were selected at random for extrapolation purposes, little was gained by making >3 counts. Nest counts are considered impractical for monitoring breeding pair densities because of the large, time-consuming samples needed; the difficulty in identifying renests; and, in redheads, the fact that some hens lay parasitically.