Code and content from my PhD thesis at Simon Fraser University in Biological Sciences. Currently undefended.
Shorebird population status and trends are commonly generated from counts made at migratory stopovers, where large numbers are concentrated at few locations. Shorebirds migrate long distances, encountering changing and unpredictable conditions. The ability to respond with adjustments in behaviour such as site selection, timing and routing, is likely essential. In this thesis I examine how the adaptive behaviour of migrants affects the use of stopover sites, and hence how many shorebirds are counted.
I develop a model of mortality-minimizing decisions made by southbound western sandpipers (Calidris mauri) moving through a landscape with large and small stopover sites. I use the model to simulate counts that would be observed under different scenarios, each leaving distinct 'fingerprints' on the outcomes. These outcomes were compared to counts made over five years by citizen-scientists across the Salish Sea region. The results support the hypothesis that inter-annual variation in the passage timing of peregrine falcons (Falco peregrinus - the most important sandpiper predator) strongly affects the distribution of sandpipers across small and large stopover sites. Other scenarios appear less parsimonious.
An analysis of data collected by the Atlantic Canada Shorebird Survey (2754 surveys, 1974 - 2015) reveals that semipalmated sandpipers (C. pusilla) have steadily shifted their stopover site usage toward larger sites. Surveys of the northbound passage of western sandpipers and dunlins (C. alpina) along the Pacific Flyway show that over recent decades (1985 - 2016), both species passage southern sites, but not northern sites earlier.
Each of these approaches demonstrates that the behavioural response of shorebirds to landscape-level conditions affects counts strongly enough that the accuracy of estimated population trends can be poor. Caution should be exerted when using migratory counts to generate trends in populations.