April Fool’s Day is the day I expect to hear the first robin sing. It’s the day that song sparrows join the chorus. Even a harsh winter does not affect their timetable much. They were a day late this year.

The night of April 2 must have been great weather for nocturnal migrants, because the fallout of birds the following day was of biblical proportions. I happened to be Down East and the roadsides were awash with thousands of sparrows and juncos. Most were song sparrows, but fox sparrows were sprinkled through the flocks. All were singing, even though none was actually upon its nesting territory. I won’t hear fox sparrows again until I get up into the northern forest.

Migration runs like clockwork. Blackbirds return around the third week of March. Turkey vultures are right behind them. The first weekend of April belongs to the sparrows and robins. The second weekend brings back the eastern phoebes. The third weekend — Kenduskeag Stream Canoe Race weekend — witnesses the beginning of the warbler wave. On the first sunny day around that time, I expect to hear a pine warbler above my house and a yellow-rumped warbler above my neighbor’s. By the fourth weekend, black-and-white warblers will lead the remaining warblers into the state.

The osprey is a conspicuous example of this timetable. There are many prominent nests on utility poles along I-95 between Pittsfield and Augusta. Shortly after April begins, I expect to see the first nest occupied. The rest fill up within days. No matter how severe the winter, the rivers are open and the birds return. Observers on Bradbury Mountain in Pownal counted 35 northbound ospreys last Monday.

The eye-catching nests along I-95 are testament to the fact that humans can sometimes recognize and correct their mistakes. Pesticides nearly wiped out the osprey, peregrine falcon and bald eagle in the 1950s and 60s. Overuse of DDT caused the chemical to build up in the food chain, either poisoning raptors outright or thinning their eggshells. By the time DDT was banned in 1972, 90 percent of the ospreys from here to New York City were gone.

Fortunately, a number of the osprey’s own traits helped bring it back. It readily nests on manmade structures. Nesting platforms were erected during the recovery period. Later, the ospreys took a fancy to bridges, towers and power lines.

The osprey also occupies a unique niche and suffers little from competition. It is the only raptor that dives into the water for fish. Eagles may be able to grab one from the surface, but they lack the ability to grab it underwater. All raptors have four toes — three in front, one behind. The osprey’s outer toe is reversible, allowing it to better hold a fish with two claws behind. I’m sure you’ve also noticed that the osprey always carries the fish facing forward for better aerodynamics.

Ospreys are highly skilled hunters. While most raptors would be happy to catch lunch on one out of ten tries, the osprey is successful on at least a quarter of its attempts, often succeeding over half the time.

Studies show that if an osprey is out fishing, he’ll bring back a fish within 12 minutes. Ospreys do take other prey, such as birds, reptiles and small mammals, but it’s rare.

Ospreys are widespread, breeding throughout the continent. Most migrate all the way to South America in the winter, though a few remain year-round in southern Florida and California. This broad distribution made recovery easier, as individuals recolonized areas where pesticides had wiped out previous generations.

Nature is cruel but efficient. Ospreys lay up to four eggs, which do not hatch all at once. It may take several days. The oldest naturally outcompetes smaller siblings, and multiple young will survive only if there is enough food. This strategy increases the likelihood of successful reproduction for at least one youngster, but survival chances for the last chick are slim.

Now here’s an identification tip. It’s often easy to tell big birds apart at great distances. Eagles, vultures, and ospreys may all look large out there on the horizon, but each holds its wings very differently.

Ospreys are noted for a sharp elbow bend, giving it the shape of an inverted gull-wing. Eagle wings are held out long and straight as a board. Vulture wings have a deep V shape. It’s obvious from a mile away.

Need another tip? The osprey is the only one with a white belly.

Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at www.mainebirdingtrail.com. Bob can be reached at duchesne@midmaine.com.