dosed. The juveniles suffered no apparent effect from the lead, whereas the adults suffered a 30 per 
cent mortality and an average weight loss of 19 per cent. 
The hen mortality from lead poisoning was found to be double the drake mortality, except in the 
spring season when hens entered the breeding phase. At this season the food intake of penned wild hens 
increased steadily until it equaled, then exceeded, that of penned wild drakes. During this period hens 
proved to be less susceptible to lead poisoning than were drakes. At all other seasons hens ate less 
food than did drakes. 
Low air temperatures encouraged a greater consumption of food by both dosed and undosed penned 
mallards, and the proportion of dosed birds surviving in winter was greater than that in milder seasons. 
In late fall and winter, mallards that died of lead poisoning did so in an average time of about 23 days 
following the ingestion of shot. 
Evaluation of the results of experiments on the influence of food revealed that the diet, rather than 
the level of the shot dose (within a range of four or fewer no. 6 pellets), was the more important variable 
in lead poisoning. 
LEVEL OF THE SHOT DOSE 
Among penned ducks on comparable diets, an increase in the shot dose beyond a single no. 6 pellet 
to a top limit of four pellets generally was followed by a corresponding increase both in the proportion 
of ducks affected and in rate of mortality. There were enough exceptions, however, to indicate that the 
range of susceptibility varied within wide limits. In some mallards, one pellet was sufficient to produce 
severe symptoms, while in others doses of one, two, three, or four no. 6’s had little or no effect. Ap- 
parently, each duck possesses an individual critical point of tolerance to lead. 
Preliminary work suggests that this critical point of tolerance may be related to the rate of food 
intake. Mallards that appeared to be fully recovered from the effects of doses of two or three no. 6’s 
were found to be eating more food than those with no history of lead poisoning. 
Among penned ducks, an increase in the dose did not increase the severity of lead-poisoning symp- 
toms, nor did it shorten the survival period of sick ducks. Post-mortem weights of poisoned ducks, re- 
gardless of the dose, were lowest during warm weather periods and highest during cold weather. 
= 121 = 
