163 
with 266 mallard nests (1,573 eggs) in 1949. Sixteen non- -mallard were under obser- 
vation in 1949, as compared with thirteen this year, The average clutch size of 8, 28 
eggs was approximately the same as last year. It is interesting to note that of the 
eggs that failed to hatch, 33.4 percent of 1,993 mallard eggs were destroyed by 
predators as compared with 18.9 percent of 996 eggs in 1949. Along with the greater 
concentration of ducks and nests, there also appeared to be an increased predator 
population on the study area, principally skunks and magpies. 
In. 1950, an average brood count of 7.4 young was obtained from 148 mallard 
nests known to have hatched, entirely or partially, and this is substantially lower than 
the 7.9 average obtained for 86 nests in 1949. Considering the 270 nests attempted, 
for which the final fate of the eggs is reasonably certain, the average number of young 
per hen was only 4.0 in 1950 as compared with 4.4 in 1949. For both years, actual 
brood counts indicated average broods somewhat lower than those derived from nest 
evidence. It appears, then, that in 1950, not only were the initial broods smaller than 
those of 1949, but a greater loss of young ducklings was suffered following the hatch. 
South Platte River Valley 
~The South Platte studies were confined to irrigated farm land and river 
bottom types for a distance of 175 miles along the valley. This 1,407 square-mile 
area was sampled in a randomized manner, the sample amounting to 4.28 percent 
(51 square miles) of the farm land and 8, 84 percent (19. 4 square miles) of the river 
bottom. The farm land production is calculated to have produced 3.77 young ducks 
per square mile in the sample areas as compared with 5. 06 in 1949, or a decrease of 
more than 25 percent; and an average brood size of 5.81 as compared with 5.18 in 
1949. River bottom land surveys indicated that 7.73 young were produced per square 
mile, as compared with 7.58 in 1949; and 5.65 young per brood as compared with 
4,83 in 1949. The data obtained is relative to all species of ducks, but mallards are 
predominant, making up more than 85 percent of all ducks in the entire area. This 
is somewhat higher than the 68.3 percent of mallards found in the valley last year. 
” The study township produced these data: 81 broods with 466 ducklings, 
averaging 5,75 young per brood and 12.95 birds per square mile. This is thought 
to be the total production this year after having completely covered the area six times 
during the season. It is interesting to note that only 39.51 percent of the total broods 
produced were seen at any one coverage. It may be possible, therefore, to derive a 
conversion factor which in turn could be applied to larger areas to determine popula- 
tions or production by a single coverage. 
Aerial Census 
The aerial census revealed 100 broods with 522 young ducks averaging 5,22 
birds per brood in the San Luis Valley. This is compared with 57 broods counted in 
1949, averaging 6.2 young per brood. 
The total adult population was about the same this year as last, the 
production increased from .24 to .43 young per square mile. 
Migration 
The extremely mild winter of 1949-50 permitted a wider use of water 
areas in Colorado early this spring than occurred in the preceding year, resulting 
in what seemed to be a less distinct migration of waterfowl. Flights of mallards, 
pintails, and baldpates apparently passed through in a much more leisurely fashion, 
extending over a longer period of time. The reduction of total water surface may 
offer a partial explanation, The numbers of migrating divers observed in April 
