"We just did some maple syruping at Camden State Park and the water was flowing in the Redwood River. There were some pools, some backwaters, holding large numbers of geese, swans and increasing numbers of waterfowl. The ducks are right on schedule. They're back about the same time they were last year. Migrating birds, they use sunlight cues from lengthening days to start their journey. Birds evolved to migrate when the length of day correlates to the availability of the food they need. There's no great way to measure migration year to year, but the 'median arrival' is one tool naturalists use. You can basically calculate the average day a species shows up for the first time. If we follow that, going by 20-year averages, we can create a list of when things should start to arrive. This week we should be seeing birds like the hermit thrush, the field sparrow and the savannah sparrow. There's a shorebird called the pectoral sandpiper — its median arrival is April 1st."
Alex Watson, regional naturalist, Minnesota DNR Parks and Trails