
drakes were in evidence from May 23 to June 3, 1951, the peak for this 
species. There was an abundance of grouped pintail drakes at this time, 
but records were kept separately for each species, and the peak in the pin- 
tail population occurred at an earlier date, before the drakes were so in 
evidence as to complicate the picture for that species. In general, other 
species followed the same pattern as that shown in table 6, and the chronol- 
ogy of the breeding season can be roughly determined from such records, 
provided they are sufficiently complete. 
Broods 
Data relating to the 1,055 broods produced on the study area from 1950 
to 1953 are shown in table 7. The first four columns show the number of 
broods per square mile actually produced each year. There was a definite 
increase in production after 1950, with a slight drop in 1953. This is par- 
ticularly true of the ruddy duck which rose from seventh place in 1950 to 
third place in 1952, with an eleven-fold increase in the number of broods 
produced. 
The four columns under "Productivity" relate production to the spring 
population in terms of broods produced per 100 pairs. 
The last three columns show production and productivity in terms of a 
hoyear average, as well as the average species composition of the broods 
produced. Here again, the blue-winged teal made up the bulk of the popula- 
tion, and the first four species made up 85 percent of the total. 
There was a great deal of variability in both production and produc- 
tivity among species and among years. Much of the later discussion will deal 
with the factors that influenced these figures. It should be remembered that 
since the figures for the breeding population represent a maximum and the 
production figures a minimum, the productivity can only be a minimum. 
Fall populations 
As indicated in table 6, the population by mid-August was increased by 
large numbers of early migrating blue-winged teal. Pintails also became 
abundant at this time, while other species migrated later in the season. The 
total number of birds on the study area was highest in late September in all 
four years of the study. Late-September populations for the four years are 
shown in table 8. Both the population size (about 23 times the number of 
young raised on the area) and its species composition indicate that the 
majority of these birds were migrants. The number of fall migrants in the 
study area appeared to vary with water conditions. In 1951 and 1953, when 
there was abundant water throughout the Dakotas and Minnesota, few birds 
went through the area; in 1950 and 1952, when the surrounding low country was 
relatively dry, many migrants used the more permanent potholes of the study 
area and the surrounding upland region. The species composition of the mi- 
grants at any particular daté depended, to a large extent, on the phenology 
of the season. In 1950, a late season, the early migrants were delayed, and 
the early migrating blue-winged teal and pintail made up a large percentage 
oh 
