An osprey feeds its chicks. (HARRY COLLINS/ISTOCK PHOTO)

This season encourages walking in the early morning hours. The air is fresh and sweet, and often filled with a variety of birdsongs. This morning I heard one of the defining calls of the area, the high-pitched tones of an osprey overhead. It is wonderful that these lovely creatures are back in the area for the nesting season. Harpswell offers ideal habitat for a bird that nests onshore but depends upon fishing for survival.

Despite nicknames like “sea hawk” or “fish hawk,” the osprey is not a hawk. The scientific name is Pandion haliaetus and ospreys may be more closely related to falcons than to hawks. Ospreys also share a trait with owls: an outer toe that is reversible, making it easier to grip slippery fish.

One of the largest birds along the coast, ospreys can reach 2 feet long. They weigh nearly 5 pounds and their wings span nearly 6 feet. At a distance, the osprey may look like a gull in flight, since both birds have arched wings, with the outer sections dipping down.

The osprey is highly efficient in flight, making use of the wind and rising thermals to glide without using much energy. This helps in migration flights that may exceed 4,000 miles, since they winter in southern states or even in Argentina. Ospreys are found in Europe, Asia, Africa and Australia, and fossils date back to 30 million years ago.

The osprey is striking, with a white chest, chocolate-brown back and wings, golden eyes, and black facial mask and talons. Males are lighter in color and have more streamlined wings and bodies. Females are, on average, 15% larger in body mass.

The hooked beak is made for rendering flesh in strips that are easily swallowed, especially by young birds. Their diet is 99% live fish, with the birds taking only the occasional small bird, rodent, rabbit or reptile. Ospreys hunt over shallow water where fish are exposed near the surface, since the osprey can only dive to about 3 feet.

After soaring up to 100 feet in the air, an osprey dives at speeds up to 80 miles per hour and plunges feet first into the water, the only raptor known to make this kind of dive. Their talons have special barbs called spicules to grip fish up to 4 pounds.

Ospreys reach sexual maturity at 3-4 years old and they often pair for life. Their nests are generally found near water, either fresh or saline. From the ground you will see what looks like an untidy pile of sticks perched high in a conifer or atop a power pole. Cornell’s Birds of the World reports that as many as 95% of osprey pairs choose man-made nesting platforms when they are available. Unlike raptors that favor secretive nest sites, ospreys are comfortable nesting near highways or residences. But if you approach an active nest, you will hear a shrill “cheereek” warning you to keep your distance.

Unless pushed out by bald eagles, ospreys renovate their nests annually, using seaweed and other vegetation for insulation. One month after breeding, the female lays two to four eggs, which hatch in about five weeks. The parents team up to feed and care for their young until the fledglings take wing at about 2 months old.

While ospreys have been known to live 25 years, a span of seven to 10 years is more typical. Thanks to the federal Migratory Bird Act, the bald eagle is the only significant predator of ospreys.

Until Rachel Carson wrote her seminal book “Silent Spring” about the dangers of pesticides, ospreys suffered from overuse of DDT. Fortunately, the birds have recovered and are still expanding their range and numbers, with the current population estimated by BirdLife at 1,500 pairs in Maine, and nearly half a million birds globally.

In recognition of their beauty, ospreys have been featured on more than 50 postage stamps around the world. It is a thrill to have these graceful birds along our shores, and before too long we will see the juveniles learning to soar on Harpswell summer breezes.

Ed Robinson’s latest book, “Nature Notes from Maine Vol. II: Puffins, Black Bears, Raccoons & More,” is available from the Harpswell Heritage Land Trust. All profits support HHLT’s conservation and public education efforts.