You may not have noticed it lurking behind the snowbanks, but spring is definitely creeping in. The birds are sure it is coming, even if you and I still have our doubts. The usual early-spring migrants–redwings, grackles, killdeer, robins–are flowing through in substantial numbers. Fox sparrows, the largest species of sparrow in North America, first showed up a couple of weeks ago, and of course song sparrows are already common.

The loud, maniacal laugh of the northern flicker is once again heard in the land, as is the soft song of the eastern bluebird. The first eastern phoebes, the earliest of the flycatchers, are already here, and in the next week or so we will start seeing towhees, yellow-bellied sapsuckers, and the earliest tree swallows. Swallows usually depend on flying insects for food, but the tree swallow also eats berries and other fruit, a habit that allows it to move north quite early.

Illinois had record numbers of wintering bald eagles this year, and some of those birds seem to be moving through the Chicago area on their way back north. Four–two adults and two immatures–were seen at Saganashkee Slough in the Palos Hills Forest Preserve recently, and another was seen at Illinois Beach State Park in Zion.

A flock of about a thousand sandhill cranes passed over the southern suburbs. They were headed northwest, probably toward their breeding grounds in central Wisconsin. The local resurgence of this species is one of the few pieces of good news about birds we have heard in these generally worsening times. After years of absence as a nesting species in Illinois, the cranes have become regular breeders at Chain o’ Lakes State Park. The large numbers we have been seeing passing through the Chicago area also suggest that the species is doing well on its long-established nesting sites in Wisconsin and upper Michigan.

Spring migration is a long and complex phenomenon. Birders have been watching it for generations now, so the patterns are quite familiar. We know what to expect and when to expect it.

The earliest birds are mostly water birds. Years ago it was discovered that geese moving north in the spring followed very closely the 35 degree isotherm. In other words, when the average temperature reached 35 degrees, you could expect the geese to arrive.

It doesn’t take a genius to figure out the connection here: 35 degrees Fahrenheit is three degrees above freezing. When the average temperature reaches that level, the ice on ponds, lakes, and rivers melts. The geese have a place to land.

March is duck month too, for the same reason. All three species of mergansers can be seen now. The two larger species, the common merganser and the red-breasted merganser, can both be found along the lakefront. The hooded merganser, a smaller bird that likes ponds and creeks, is usually seen inland.

I love mergansers. They are gorgeous birds, startlingly patterned and colored. The red-breasted merganser and the common merganser female have wonderfully shaggy crests that make them look like they just stuck their beaks in a light socket.

In flight, mergansers look streamlined and sleek, without the dumpy build of other ducks. This profile is related to their way of life. They are the only ducks that catch fish. Other ducks are either vegetarians or eat animal food but go after small crustaceans, insect larvae, and other slow-moving stuff. You have to be quick and maneuverable to catch a fish. Mergansers look more like miniature loons than ducks.

In winter we usually see the common merganser on the lake. As spring approaches, it leaves and the red-breasted replaces it. When the smelt are running, hundreds of red-breasted mergansers gather in large flocks offshore.

Both of these large species nest north of here, but the hooded merganser sometimes nests in Illinois. In fact, a pair nested in Jackson Park as recently as 1980.

We are also seeing a few canvasbacks and some redheads. Canvasbacks have always been the favorite duck of hunters because their meat is supposed to be tastier than that of any other species. It is a diving duck, primarily a vegetarian, and it prefers the roots and tubers of aquatic plants. Its fondness for a plant called wild celery (Vallisneria americana) caused the early American ornithologist Alexander Wilson to give it the scientific name Aythya valisineria. (You may notice that Wilson’s spelling of the species name is incorrect. According to the rules of taxonomy, the oldest name for the species has priority over later names, so Wilson’s poor spelling must be retained.)

Canvasbacks often strain their food from bottom mud, a habit that has made them especially vulnerable to lead poisoning from shot buried in the mud of marshes that have been heavily hunted over.

Ducks are notoriously loose in their sexual habits. Hybrids between various species are quite common. Drakes of one species will court females of another species, and the females often respond. And the shocking irregularities don’t end there. Some ducks lay their eggs in the nests of other species. Redheads do this fairly often. They also build their own nests, but they tend to be rather casually attached to them. And they abandon their young at an earlier age than most ducks.

Pied-billed and horned grebes are also arriving about now. Grebes are swimming birds with an extraordinary ability to control their buoyancy. At the approach of danger, a swimming grebe can simply sink until it is completely out of sight. Or it can sink until only its head is above water. From that position it can keep an eye on things while presenting a shape that looks nothing like a bird.

The songbirds that migrate early tend to be ground feeders like robins, bluebirds, grackles, and meadowlarks. Soon we will be seeing birds like the brown creeper and our two species of kinglets: golden-crowned and ruby-crowned. These birds are insect eaters and, especially, eaters of insect eggs. They glean their food from the trunks and branches of trees.

If you keep your eyes open, you are bound to see brown creepers. They will show up on any street where there are trees. Look for a small brown bird hitching its way up a tree trunk, circling the trunk as it rises. When it reaches the first branches, it will probably fly to the base of a nearby tree and start the process over again. This is a bird you can identify easily without binoculars.

Birds of prey can be found throughout the season. I mentioned the bald eagles that have come through. Harriers are also around now, and the last rough-legged hawks are now leaving for the tundra. The last migrant among the raptors is the broad-winged hawk, a woodland species that winters in South America and passes through here in May. Huge flocks of these birds can be seen on good days. They drift slowly northward, high in the sky, riding thermal currents.

Migration reaches its peak in May–at this latitude about mid-May. There are more species and more individual birds to be seen then than at any other time of the year. The insect-eating songbirds–warblers, vireos, tanagers, flycatchers, swallows–are the most numerous.

These birds move north in sync with the opening of leaves in the spring. Of course, the opening of leaves also signals the emergence of the insects that eat the leaves. Birders tend to like cool springs when the leaves open slowly and the warblers are more easily seen. If the leaves open fully early in May, you can spend some very frustrating times trying to make out the identity of a flitting shape totally hidden by the foliage.

The passing of the shorebirds brings an end to the migration season. These sandpipers and plovers nest mostly way up north on the tundra, and their movements are timed to allow them to arrive on their nesting grounds after the northern insect populations have emerged.

Bird migration is one of the most wonderful spectacles in the natural world. It is not what it once was. Populations of many species are far below the levels of even 20 years ago. But the show is very easy to see even here in the city. In fact, you have to practically close your eyes until the middle of June to miss it.