Spring migration is in full swing. Excitement among birders is at fever pitch. Each day has the potential to bring in a new species.

Try to get out each morning for the next week or so to experience the spectacle of the spring migration. Birds will be in full breeding plumage and song will be in the air.

When weather conditions are right, hordes of migrating birds may descend shortly before dawn. A fallout occurs and birds seem to be dripping from the trees and bushes.

You can literally see a fallout occur through the use of radar. Bird images can be detected on the freely available radar plots. See this video (vimeo.com/2020985). Using radar images, you can see a big flight of birds taking off in the early evening and then be on the ground the next morning.

The spring migration is the time when some competitive birders will undertake a Big Day. A team of birders seeks to identify as many species of birds as possible in a 24-hour period in a state, county or other area.

A successful Big Day has to be timed around migration. To understand why, it is convenient to sort our migrants into three categories. Wintering migrants are birds that winter here but depart in the spring for more northerly breeding grounds. American tree sparrows, snow buntings and Lapland longspurs are good examples.

Then we have the breeding migrants. These are the species that winter to our south but return to Maine to breed.

Some of these breeders are short-distance migrants like American woodcocks, red-winged blackbirds and yellow-rumped warblers that may overwinter as far north as the mid-Atlantic states. None leave North America for the winter.

Other breeding species leave North America to winter in the West Indies, Central America and South America. We refer to these birds as neotropical migrants or long-distance migrants. Familiar examples are broad-winged hawks, ruby-throated hummingbirds, cliff swallows, red-eyed vireos and most of our warblers.

The third category of migrants are the passage migrants. These birds winter to our south and breed to the north of us. We see them only fleetingly as they migrate through Maine en route to nesting or wintering territories. The white-crowned sparrow is one such passage migrant from the Maine perspective. These handsome sparrows winter broadly throughout the southern two-thirds of the United States. They pass through Maine on their way to northern Canada, where they breed. We get to see them again in the fall as they head for points south to winter. A number of shorebirds are passage migrants as well.

The passage migrants are the species that make or break a successful Big Day. The day has to be chosen to try to maximize the number of passage migrants that can be found to augment the expected resident species, migratory breeding species and lingering winter residents. For Maine, that sweet spot seems to be the third week of May.

The northerly and southerly paths of passage migrants may vary. The semipalmated sandpiper is a case in point, breeding on the North American Arctic tundra and wintering in the region of the Amazon delta in South America.

Most semipalmated sandpipers migrate north through the center of North America with a major stop in Cheyenne Bottoms, Kansas. Relatively few migrate along the Atlantic seaboard.

After breeding, most semipalmated sandpipers work their way to the Bay of Fundy. We see many more of these sandpipers in the fall in Maine.

Why the difference? As is often the case with birds, food drives behaviors. The small crustaceans that sandpipers feed on in the fall in the Bay of Fundy are at low abundance in the spring but rebound over the course of the summer. Semipalmated sandpipers have to find a more reliable source of food in the spring migration, and the marsh insects of middle America fit the bill.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

whwilson@colby.edu