A Method for Investigating Population Declines of Migratory Birds Using Stable Isotopes: Origins of Harvested Lesser Scaup in North America
Figure 2
Geographic distribution of assigned sites of origin for hatching year scaup harvested in all flyways in 1999 and 2000 (n = 867) inferred from stable isotope (δDf) analysis of feathers.
Inset depicts the flyway boundaries. Assignments were based on a Bayesian framework incorporating a spatially explicit isotope-based likelihood function with spatially explicit prior probabilities of being shot in a given flyway (based on banding records). The posterior probability density for each individual was then converted to a binary surface by assigning a “1” to all grid points with odds greater than 2∶1 relative to the highest density value and a “0” to all others. Each grid point shown here represents the sum of these individual-level binary grids, summed over all individuals across all four flyways and both years. Thus, a value of 177 represents a cell being consistent with representing an origin for 177 birds within the sample for example.