Conventional 35 mm photography was used to conduct an aerial photographic census of canvasbacks (A. valisineria) throughout Chesapeake Bay (tidal Maryland and Virginia) and coastal North Carolina, Jan. 26-30, 1981. Flock size and sex ratio characteristics were determined from examination of color transparencies of 165 canvasback flocks totaling > 95,000 birds. A sex ratio of 2.91 males/female was determined from 68,769 birds, 80% of the birds in 150 flocks. Sex ratio for the Atlantic Flyway was projected as 2.90 males/female. The greatest number of canvasbacks and the widest range of flock size were recorded in Maryland waters; the fewest canvasbacks and the smallest average flock size in Virginia; and the fewest but on average the largest flocks of canvasbacks in North Carolina. Sex ratio varied latitudinally in the flyway with a tendency for males to occupy more northern and females more southern latitudes in winter. Sex ratio (males/female) was highest in Maryland (3.98), slightly lower in Virginia (3.71), and lowest in North Carolina (1.70). Locally, sex ratio varied with flock size. In Chesapeake Bay, small flocks (< 100 birds) had lower sex ratio (P < 0.05) than medium (100-1000) or large (> 1000) flocks. By providing large-sample sex ratio information, as well as exact counts of birds, low-level 35-mm aerial photography is the most efficient and accurate means of determining canvasback population status in eastern coastal habitats.
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An aerial photographic census of Chesapeake Bay and North Carolina canvasbacks