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Environmental Characteristics Associated with the Occurrence of Avian Botulism in Wetlands of a Northern California Refuge

Study Area

Our study was conducted at SNWR, near Willows, California. The refuge comprises 2,995 ha of intensively managed wetlands and is an important area for many species of migratory birds, especially waterfowl. This site has a documented history of Type C avian botulism outbreaks in waterbirds since 1958, typically occurring during late July through October (J. G. Mensik, SNWR, personal communication), and Type C botulinum spores were found to be highly abundant in SNWR wetlands (Sandler et al. 1993). We selected 10 wetlands for our study, ranging from 4 to 75 ha in size: 4 wetlands were flooded year-round and 6 were flooded seasonally (Sep–Apr). Vegetation varied among wetlands, but dominant plant species included hardstem bulrush (Scirpus acutus), cattail (Typha spp.), swamp timothy (Crypsis schoenoides), smartweed (Polygonum spp.), and cocklebur (Xanthium strumarium).

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