http://www.duckdvm.com/condition/botulism
From the article:
How do Ducks Get Botulism?
Ducks can get botulism through eating or dabbling in contaminated soil, water, decaying matter, spoiled feed, or from eating maggots harboring C. botulinum toxins. Decaying animals or vegetation provide both a protein source as well as an anaerobic environment to C. botulinum. Botulism Type C spores exist along the bottoms of various water sources, which begin to thrive as oxygen levels drop and water temperatures rise.