associated with marshes and mostly from blinds, with little open- 
water, point, or pass shooting. 
On Mark Twain Refuge field feeding is the rule, either on 
refuge lands or in agricultural fields away from the refuge. 
Again, it is surmised that this is due to ducks moving in from a 
northwesterly direction from the prairies, where they have ex- 
perienced field feeding to a large extent. 
The Bureau has little control over water-level management 
except for a few, small, diked-off areas on the Upper Mississippi 
Refuge, and on Louisa and Keithsburg Units of Mark Twain Refuge. 
The Batchtown and Calhoun Units are both backwater areas result- 
ing from impoundment in connection with the 9-foot navigation 
channel project. The Gardner Unit is a huge island in the 
Mississippi River, a large portion of which has been farmed for 
many years. Elsewhere, the Corps of Engineers controls water 
levels primarily in the interest of navigation. So far as 
possible, the Corps manages water levels during the fall with 
both waterfowl use and hunting opportunity in mind. 
TECHNIQUES 
Waterfowl populations 
The Upper Mississippi Refuge is divided into administrative 
districts, each covering a different segment of the river and 
averaging about 30 miles in length. A resident manager is 
assigned to each district. It is his responsibility to conduct 
inventories and bag checks and to determine hunter activity in 
his own district. Each week a ground census is conducted by the 
district managers, and the data are submitted to the Winona office 
where they are compiled to determine the total refuge population. 
This system of obtaining population data has been in use for the 
entire period of this study, and it has ensbled headquarters to 
keep quite accurate records on waterfowl movements and popula- 
tions. Since 1955, in addition to the ground surveys, biweekly 
aerial surveys have been conducted each fall. Reconciliation 
is made between the air and ground data. 
Resident managers have also been assigned to each unit of 
Mark Twain Refuge, and they too have conducted surveys and re- 
ported weekly populations and movements. In addition to these 
ground counts, we also receive reports from Frank Bellrose of 
the Illinois Natural History Survey who conducts periodic aerial 
surveys which include the Mark Twain Units. 
