Many birders mark the progress of spring by noting migrants’ dates of arrival. A woman I talked to in Blue Hill, for instance, kept her eye on a particular osprey nest. She remarked that its owners would show up within a day or two of April 15 each year – an excellent antidote for the terror of Tax Day!

Ospreys are very faithful to their nesting sites. They will continue to return to the same location each year, renovating or rebuilding a nest as necessary. If they are unsuccessful in producing young at a particular nest, though, they will pick another spot.

These birds are quite numerous in Maine and don’t seem to mind nesting near humans as much as other raptors do. As a result, their courtship and territorial activities are easy to observe; the most arresting of these is the “sky dance.”

The male performs this maneuver soon after arriving in his territory.

Ascending steeply with rapid wing beats to a height of several hundred feet, he hovers with fanned tail and hanging talons. He then makes a spectacular dive, stalls, and swoops up again to hover. This aerial roller-coaster ride is most often done above the nest, and serves both to advertise his territory and attract his mate. He may hold nesting material or a fish in his talons while he does this.

Later in the season, ospreys – or “fish hawks” – may perform this exercise over their fishing grounds.

The osprey is similar to the bald eagle in that it doesn’t mature until its later years. Unlike the eagle and other raptors, however, immature ospreys remain on their wintering grounds in South America instead of migrating back up north.

The year after that they return to the place of their birth, where they may pair up and start to build a nest. This is just “for practice;” they do not actually breed until they reach 3 years of age.

Ospreys specialize in capturing fish almost exclusively; specific adaptations allow them to do this. Long legs with short, stiff feathers make it easier for the bird to thrust its feet into water to secure prey; dense, oily feathers help prevent the bird from becoming waterlogged; and spiny scales on its talons ensure that its slippery prey doesn’t wiggle out of its grasp.

The northeastern population of ospreys was nearly wiped out by DDT in the 1960s. Thankfully, the banning of the pesticide, and efforts to increase success by providing nesting platforms and viable eggs from other birds, enabled it to make a comeback.

Chris Corio’s column on birds is published each Saturday. Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com