are based on birds seen, and it is known that when making aerial observations 
a portion of the birds are missed. Even though the "index" figures are not a 
measure of total populations, it is believed that they are representative of 
relative population levels to the extent that data from one location can be 
accumulated with those from another, and that year to year changes can be 
detected. Although a measure of total population would have certain advantages, 
a determination of relative changes seems adequate for the purpose of practical 
management. 
Needless to say, the breeding ground surveys are cooperative in 
nature. The Fish and Wildlife Service, the Canadian Wildlife Service, the 
Provincial Game Branches, and Ducks Unlimited combine their manpower 
and equipment to cover all of the important waterfowl breeding areas in Canada, 
Service Biologists cover the important areas in Alaska, while the State 
Conservation Agencies, with some help from the Service, carry on surveys in 
about 25 States. 
