Spring
The busiest birding season begins in April, when blackbirds, waterfowl, sparrows, and the first warblers return. Expect mud and standing water. Migrants arrive in Southern Maine and Portland about a week ahead of Acadia and Bangor, which in turn are nearly a week ahead of the Katahdin and Aroostook regions. Snow clears from the eastern coastal areas earlier than the western mountainous regions.
On average, by April 1st, American Robins and Song Sparrows have returned and begun singing. Most rivers are flowing free and open, attractive to diving ducks including Barrow's Goldeneye. By the second week of April, ice on the lakes and ponds has begun to disappear. A great variety of ducks move into open water and flooded farm fields at their first opportunity. Purple and House Finches also reappear in greater numbers about this time.
By the third weekend of April, Tree Swallows and the earliest warbler species begin to show up, specifically: Yellow-rumped, Pine, and Palm Warblers. Calls of the first Eastern Phoebes and Yellow-bellied Sapsuckers liven up quiet mornings in late April. Hawks also begin to return around this time. Many of Maine's wintering ocean ducks use the Kennebec and Penobscot Rivers, as well as the Allagash and St. John rivers further north, to reach Hudson Bay. It's common for eiders, Buffleheads, and scoters to show up on rivers and adjacent lakes in migration through mid-May.
Spruce Grouse are often easiest to encounter before foliage thickens. April can be a good time to see them. April can be a good time to see them.
Summer
Around the first of May, the remaining warbler species begin to return, peaking by Memorial Day. Ruby-throated Hummingbirds can be expected around Mother's Day. Some migrants such as Nelson's Sparrows and Black Terns may not return until early June.
As an example of geographical diversity, Maine Audubon Big Day teams typically achieve species counts around 140 in southern Maine and about 125 in eastern and central Maine, since Portland is the northern limit for some species. Warbler season is richest between mid-May and mid-June. Black-backed Woodpeckers are noisiest around the nest in late May and early June. Boreal Chickadees are noisy anytime, EXCEPT when nesting during the last two weeks of June.
By July the woods are more quiet as the parents raise their broods. In August, southward migration begins again, starting with shorebirds. Though most shorebirds use the ocean beaches and marshes, a surprising number can be found on inland river and lakeside mudflats. Peregrine Falcons and Merlins follow the shorebirds so either may be encountered at these places. Pelagic activity picks up dramatically in August. Wilson Storm-petrels and Great, Sooty, and Manx Shearwaters increase. Atlantic Puffins, Common Murres, Thick-billed Murres, and Razorbills disperse to open water. Jaegers mix with gulls and Northern Fulmars.
This activity peaks in late September, though Northern Gannets straggle southward from late July through Christmas.
Offseason
The bulk of hawk migration occurs around the middle of September, and in October the sea ducks start to return from Hudson Bay. By November, some species that breed in Canada irrupt into northern New England. Common (and sometimes Hoary) Redpolls, Pine Grosbeaks, White-winged and Red Crossbills, and Bohemian Waxwings may turn up everywhere after an absence of several years. Snow Buntings may be common or scarce in any given winter.
Snowy, Great Gray, and Northern Hawk-owls are all rare, but in the right winter, they appear in Maine.
In November and December, wintering sea ducks flood the coasts. Common (and occasional King) Eiders, Long-tailed Ducks, Buffleheads, Common, Red-throated, and Pacific (rare) Loons, Black Guillemots, Common Goldeneyes, Horned and Red-necked Grebes, and all three scoters are findable just about anywhere. Harlequin Ducks are regular in several places and uncommon in several more. Purple Sandpipers are common for the experienced eye.
January-March (winter): Bird anywhere along the coast for sea ducks. Look for freshwater ducks in the open water below
dams. Snow Buntings and sometimes Horned Larks frequent beaches, particularly at Reid State Park and Popham Beach State Park in the Midcoast area. A Lapland Longspur is possible in such flocks, too. Northern Shrikes may turn up anywhere in winter but enjoy the edges of open areas along the coast and farmlands.