Accurate estimates of total mortality in outbreaks of avian botulism are diffi- 
cult, and the number of dead birds reported in a single outbreak has often varied 
greatly. Most figures appearing in the literature must be considered with this point 
in mind. However, counts of individual carcasses over limited areas, such as on 
individual units at the Bear River Refuge in Utah, give more accurate information. . 
Some rough estimates of mortality in the enormous outbreaks in the vicinity of 
Great Salt Lake have placed the total number of dead at around 300,000 birds in a 
single year. During one of these serious years, workers picked up and buried nearly 
47,000 carcasses from what must have been a relatively small area. In 1932, Kalmbach 
(1934) estimated that fully 250,000 birds perished in the Bear River Bay and Willard 
Spur sections at the northern end of the Great Salt Lake. 
Actual collections of sick and dead birds from Unit 2 (5,000 acres) of the Bear 
River Refuge for 1943, 1944, and 1945 totalled 510, 1730, and 1926, respectively. 
Most of these were found along the main dikes and principal waterways. Additional 
thousands were affected and noted in distant areas where collections were not made. 
Since the development of water-manipulation practices for prevention of botulism, 
_mortality on the refuge itself is reported to have been lessened, though it is still 
at a widely variable figure from year to year. During the season of 1951, relatively 
few deaths occurred there in the early part of the usual danger period, but later many 
dead were encountered both within and without the refuge. 
The species of ducks principally affected by avian botulism are those that have 
the greatest number of individuals utilizing critical areas during the botulism season. 
Reliable data show that in the Bear River region the main species affected are the 
pintail and the green-winged teal. These two species represent the bulk of the pop- 
ulation present at Bear River during the botulism season. Present in smaller numbers, 
and having lower total mortality, are mallards, shovelers, cinnamon teal, baldpates, 
gadwalls, redheads, blue-winged teal, and ruddy ducks. 
Reports from observers in other areas of this continent have added various other 
species of ducks to the list of botulism victims. These are of lesser importance in 
the total picture of the malady as they compose an extremely small portion of the total 
population exposed to toxicity. Included are ring-necked ducks, canvasbacks, lesser 
and greater scaups, fulvous tree ducks, black ducks, and others. Grebes, geese, 
mergansers, and coots sustain a less heavy mortality from botulism than the puddle 
ducks. This may be due in part to their different feeding habits. Gulls, shorebirds, 
waders, and other water-frequenting species are known to be affected. Shorebird losses 
are sometimes heavy. 
If the immense botulism mortalities recorded in the past half-century represent 
an up-swing in the disease, this conceivably may be related to changes induced by agri- 
cultural development of the West. As irrigation became widespread, it was accompanied 
by lowered water tables and a recession of waters in most of the great shallow alkaline 
basins. This recession of shorelines often exposed great expanses of lake-bottom mud 
susceptible to seasonal or wind reflooding. At the same time, other marsh and lake 
areas dried completely, causing increased concentrations of waterfowl populations. 
Thus, in recent times conditions may have become more conducive to the development of 
great epizootics. 

S52 
