Fifty years ago, red-bellied woodpeckers were regarded as vagrants in Maine. Now they are established nesters in the state. Julio Cortez/Associated Press

The bird checklist for Maine currently stands at 475 species which includes resident birds who grace us with their presence all the year, like downy woodpeckers, American crows and black-capped chickadees.

Over 100 species are migratory breeding birds, moving from wintering areas to our south to nest here and then departing before winter sets in. Killdeer, eastern phoebes and yellow warblers fit the bill. We look forward every spring and fall to passage migrants – birds that breed to our north and winter to our south – such as plovers and sandpipers are passage migrants.

Some species on the Maine list are irruptive species, northern species that move south to winter in Maine when their food at high latitudes is hard to find. Although we can’t depend on seeing them every year, we can consider species like snowy owls, common redpolls and pine grosbeaks to be expected members of the Maine avifauna.

The remainder of Maine bird species are vagrants, birds that only occur rarely and unpredictably in Maine, and some like white-eyed vireo and hooded warbler occur annually in Maine. There are also 107 species in Maine that have been sighted five or fewer times, 49 that have been seen only once and another 29 only twice. The sighting of such a bird leads to an onrush of birders anxious to see one of these out-of-range birds. But how did they get here in the first place?

Some birds like swans, geese and cranes migrate as family units, so young birds are shown a migration route by their parents. Most songbirds and some other groups of birds have their migration direction encoded in their genes. A first-year ruby-throated hummingbird or black-and-white warbler finds its way to the species’ wintering ground on its own.

Genes, however, are subject to mutation. Mutated migration genes may direct the bird in the wrong direction. In most cases, the direction is 180 degrees opposite of the correct direction. Such reverse migration can lead a bird to head north instead of south or east instead of west. Less commonly mutations may misguide a bird, but not in the exact opposite direction.

Weather can disperse birds to areas beyond their normal range. A hurricane or tropical storm can spirit seabirds or coastal birds long distances. Strong winds can bring gull-billed terns or black skimmers to Maine from southern coastal areas.

Hurricanes do not play much of a role in transporting landbirds, most of which hunker down before a hurricane hits. But winds associated with weather fronts can explain some vagrancy. Songbirds migrating south from areas to our south on favorable winds may encounter a front with strong winds blowing to the northeast. Those poor birds are buffeted northward until they can find landfall. The slight easterly direction of those winds means that birds are sometimes blown out to sea. It’s no wonder that places like Monhegan Island, Matinicus Rock, Mount Desert Rock and Petit Manan Island have so many extraordinary records of southern vagrants.

During spring migration, some species that nest just to our south may overshoot and land in Maine. They may linger for a day or two but then correct their overflight by flying south to their normal range. We can usually count on a few hooded warblers, worm-eating warblers or summer tanagers every spring.

Some species engage in post-breeding dispersal. Egrets, herons and ibises are particularly known for this behavior. One should never be surprised to see an egret or glossy ibis inland at this time of year. Just last year, a flock of white ibises visited Wells in August.

These birds may be prospecting for good future nesting sites. In the 1970s the northern breeding limit of glossy ibis was in North Carolina in the Morehead City area. This species has expanded its nesting range to southern Maine. I can’t help but think that post-breeding dispersal facilitated this rapid range extension over the past 50 years.

Finally, some species expanding their ranges first trickled into Maine only to become established breeders. This expansion is driven by either population increases in source populations or by climate change. Fifty years ago, turkey vultures, red-bellied woodpeckers, tufted titmice and Carolina wrens were regarded as vagrants. Now they are established nesters in the state.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu