Stopover duration of fall-migrating dabbling ducks

The amount of time migrating birds spend at stopover sites, or stopover duration, partially determines an individual's access to resources, the environmental conditions encountered, and the exposure to predation, which in turn affect survival and fecundity. As such, migratory behaviors such as stopover duration can have a considerable effect on populations of migrants and plans for their conservation. This is especially true for migrant waterfowl, which are explicitly conserved through Joint Venture (JV) partnerships under the North American Waterfowl Management Plan. Although waterfowl are one of the most heavily studied taxa, little is known about their stopover behavior due to the scope of migration. Consequently, conservation plans of many mid-migration JVs either omit estimates of stopover duration or rely on antiquated data to estimate energetic requirements. We used weather surveillance radar to identify and enumerate ducks emigrating from an important stopover area in central Illinois. By using radar data in combination with data from weekly aerial inventories, we estimated an average stopover duration for fall-migrating dabbling ducks (tribe Anatini) of 28 days (SDa =a 12) over 8a years (1996, 1997, 2003, 2005-2009). Our estimate was similar to the historical estimate of 28 days (1940-1966), which serves as the primary reference for the Upper Mississippi River Great Lakes Region JV conservation plan. In addition to a corroborative mean, we also found considerable inter-annual variation in stopover duration. Estimated annual stopover duration was correlated positively with an index of annual foraging habitat quality (Spearman's rank correlation; r _sa =a 0.83), suggesting ducks may have assessed local conditions and adjusted the spatiotemporal course of fall migration. If the stopover behavior of fall-migrating ducks is flexible and forage-dependent, it is possible ducks allocate their time among sites in a somewhat ideal and optimal fashion, which could substantially affect the way resources are allocated within the spatial context of a JV region.