Relationships between local carcass density and risk of mortality in molting mallards during avian botulism outbreaks.

Removal of bird carcasses has been advocated for management of Clostridium botulinum outbreaks on lakes in North America because a reduction in density of toxin-laden maggots produced within bird carcasses is assumed to enhance survival of healthy birds. This inverse relationship between carcass density and survival has been reported in controlled studies with Mallards (Anas platyrhynchos) but has not been verified in wild ducks during naturally occurring botulism outbreaks. Therefore, we radio-marked 204 molting Mallards on seven lakes in western Canada during July-August 1999-2000, and monitored their survival daily for 30 days. Carcass searches were conducted simultaneously at 90 matched locations for freshly dead and randomly selected live radio-marked Mallards. Carcass density (carcasses/ha) averaged about two times greater at dead than at live duck locations (x = 12.4, SE= 1.2 vs. x= 5.0, SE= 0.7). Predicted risk of mortality increased rapidly with carcass density (case-control logistic regression: model-averaged beta(density)= 0.167, unconditional SE= 0.062). Mallards exposed to 5-11 and >11 carcasses/ ha were 3.5 and 13 times more likely to die, respectively, than were Mallards inhabiting carcass-free areas. Mortality risk was more closely related to density of maggot-laden carcasses than to maggot-free carcass densities. Our results are consistent with the assumption that reducing carcass density could enhance survival. However, we caution that survival rates may remain low on lakes in which areas with high carcass densities persist due to incomplete carcass removal.

Survival of radio-marked mallards in relation to management of avian botulism.

Avian botulism outbreaks are frequently perpetuated by type C toxin produced by Clostridium botulinum proliferating in decomposing bird carcasses and consumption of toxic maggots from these carcasses by healthy birds. Therefore, removing bird carcasses has been advocated for disease management because availability of toxic maggots should be reduced, increasing duck survival. However, this management is expensive, and its effect on waterfowl mortality under field conditions is unknown. We radio-marked 419 molting mallards on 11 lakes in western Canada during July-August 1999-2001 and monitored them for 30 days, testing whether survival was higher on lakes with carcass removal. Botulism occurred on 10 lakes. On five carcass removal lakes, greater-than-normal effort was made to conduct early, thorough surveillance and immediately remove carcasses; on six nonremoval lakes, no carcasses were removed. In 1999, estimated 30-day survival probabilities ranged from 0.149 (95% CI=0.065-0.304) on one large lake with carcass removal to 0.466 (95% CI=0.270-0.674) and 0.618 (95% CI=0.443-0.767) on two nonremoval lakes. As a result, we conducted work on smaller wetlands thereafter, reasoning that any management benefit would be easier to detect. In 2000, estimated 30-day survival probabilities were 0.313 (95% CI=0.143-0.556) and 0.794 (95% CI=0.609-0.905) on two carcass removal lakes versus 0.525 (95% CI=0.362-0.682) and 0.743 (95% CI=0.564-0.866) on two nonremoval lakes. In 2001, botulism was detected on two nonremoval lakes where survival probabilities were 0.845 (95% CI=0.630-0.946) and 0.942 (95% CI=0.778-0.987), and on one removal lake where survival probability was 1.0 (95% CI=0.99-1.0), but not detected on the other removal lake where no marked birds died from botulism (1.0, 95% CI=0.99-1.0). Survival tended to be higher on lakes with lower carcass density, but when data were organized by carcass removal versus nonremoval, mallard survival was not consistently greater on lakes where carcasses were removed.