Counts of wood ducks (Aix sponsa) made during canoe float trips on Mississippi River tributaries in northeast Iowa were less useful than counts made at mouths of those streams as flights passed from scattered roost sites in the bottoms to nesting, feeding, and loafing areas in tributary valleys. Coefficients of variation were 0.29 for float counts and only 0.08 for flight counts. River float counts and flight counts of wood ducks were made in northeast Iowa during April, 1962. This paper compares the usefulness of these two methods for estimating abundance of nesting wood ducks. River float counts are widely used to measure breeding populations and production of wood ducks. However, there is disagreement concerning reliability of this method. Mumford (1952) conducted a 3-year study of river float counts in Indiana and concluded that they gave usable estimates of wood duck production when made under favorable conditions. In Ohio, Stewart (1958) found that river float counts made with one trip in each of the three months, April, May, and June, were not sufficiently sensitive to show anything but major changes in numbers. He added that if all three trips were made within a short period, a measurement of variation could be obtained. Martin (1959) discussed spring and summer float counts and concluded that the technique was unsuitable in Iowa. Hein (1962) found that fall float counts were unreliable in Iowa owing to variation in counts according to time of day, length of float trip, and nature of the stream. Wood duck flight counts have not been used during spring, although fall roosting flight counts have been investigated by Smith (1958), Stewart (1958), Martin (1959), and Hein (1965). Data were collected along the Mississippi River in northeast Iowa. Most of the bottomland habitat was part of the Upper Mississippi River Wildlife and Fish Refuge administered by the Bureau of Sport Fisheries and Wildlife. The bottoms were 1 to 3 miles wide and consisted of wooded islands, sloughs, and marshes interspersed with open 1 Journal Paper No. 5089 of the Iowa Agricultural and Home Economics Experiment Station, Ames. Project No. 1391. A contribution from the Iowa Cooperative Wildlife Research Unit, U. S. Bureau of Sport Fisheries and Wildlife, Iowa State University, Iowa State Conservation Commission, and the Wildlife Management Institute, cooperating. 2 Present address: Department of Biology, Wake Forest College, Box 7325, Reynolda Station, Winston-Salem, North Carolina. 13 JORDAN, J. S. 1953. Consumption of cereal grains by migratory waterfowl. J. Wildl. Mgmt. 17(2):120-123. MARTIN, A. C., H. S. ZIM, AND A. L. NELSON. 1951. American wildlife and plants. Dover Publications, Inc., New York. ix + 500pp. McFARLAND, L. Z., H. GEORGE, AND H. MCKINNIE. 1963. Grain preference of captive waterfowl. California Fish and Game 49(3):207-209. MoRRISON, F. B. 1956. Feeds and feeding. 22nd ed. The Morrison Publishing Company, Ithaca, New York. vi + 1165pp. Received for publication August 4, 1965. This content downloaded from 40.77.167.34 on Fri, 01 Apr 2016 05:05:30 UTC All use subject to http://about.jstor.org/terms 14 Journal of Wildlife Management, Vol. 30, No. 1, January 1966 water. Wooded bluffs rose as high as 600 feet on each side of the floodplain. Numerous tributary valleys were important extensions of habitat for wood ducks. Before the incubation period of the nesting season, most pairs roosted each night at scattered locations in the Mississippi River bottoms. At dawn more than half of these birds left the bottoms and entered tributary valleys and ravines to spend the day searching for nesting cavities, egg laying, feeding, and loafing before returning to the bottoms at dusk to roost. These flights became unpredictable after hens began incubating.