FLOCK SIZES AND SEX-RATIOS OF CANVASBACKS IN CHESAPEAKE BAY AND NORTH-CAROLINA

Knowledge of the distribution, size, and sex ratios of flocks of winte ring canvasbacks (Aythya valisineria) is fundamental to understanding the species' winter ecology and providing guidelines for management. C onsequently, in winter 1986-87, we conducted 4 monthly aerial photogra phic surveys to investigate temporal changes in distribution, size, an d sex ratios of canvasback flocks in traditional wintering areas of Ch esapeake Bay and coastal North Carolina. Surveys yielded 35-mm imagery of 194,664 canvasbacks in 842 flocks. Models revealed monthly pattern s of flock size in North Carolina and Virginia, but no pattern of chan ge in Maryland. A stepwise analysis of flock size and sex ratio fit a common positive slope (increasing proportion male) for all state-month datasets, except for North Carolina in February where the slope was l arger (P < 0.001). State and month effects on intercepts were signific ant (P < 0.001) and confirmed a previously identified latitudinal grad ient in sex ratio in the survey region. There was no relationship betw een flock purity (% canvasbacks vs. other species) and flock size exce pt in North Carolina in january, February, and March when flock purity was related to flock size. Contrasting characteristics in North Carol ina with regard to flock size (larger flocks) and flock purity suggest ed that proximate factors were reinforcing flocking behavior and possi bly species fidelity there. Of possible factors, the need to locate fo raging sites within this large, open-water environment was hypothesize d to be of primary importance. Comparison of january 1981 and 1987 sex ratios indicated no change in Maryland, but lower (P < 0.05) canvasba ck sex ratios (proportion male) in Virginia and North Carolina.