Migration as flow

Using hydrological concepts to estimate the residence time of migrating birds from the daily counts

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Estimating the length-of-stay, the number of days a bird can be expected to stay at a site, at stopover sites is critical to understanding the migration ecology and estimating the population sizes of birds as they move between breeding and non-breeding sites. Estimating the length-of-stay of migrating animals at stopover sites has an analogue in the hydrological concept of transit time, the amount of time that water spends in a reservoir, which can be calculated as a numerical integration of inflow and outflow rates with an underlying Storage Age Selection function. We used this approach to estimate the lengths-of-stay of migrating Western Sandpiper (Calidris mauri) and Dunlin (Calidris alpina) based on the time series of daily counts at two sites in British Columbia, Canada. The approach yielded mean transit times for Western Sandpiper during southward migration at Sidney Island that ranged between 9·6 days and 3·8 days, and showed a significant decline over time, 1992-2001, and is consistent with the estimates obtained from the capture-mark-resight studies. Transit times during northward migration at Roberts Bank, Fraser River Delta, based on the best available information ranged from 1·8 to 3·2 days for Western Sandpiper, and had a median value of 2·0 days for Dunlin, which is consistent with the estimates obtained from the radio-telemetry studies. These results indicate that the hydrological flow models may offer a means to estimate the length-of-stay from the daily counts of birds during migration. The models present an opportunity for testing the alternate hypotheses concerning the roles of behavioural- vs. habitat-related mechanisms driving shorebird population sizes.