An aerial photographic census of Chesapeake Bay and North Carolina canvasbacks

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.