Common loons are well-known and beloved in the northern Great Lakes and much of Canada, where their eerie calls echoing across the lakes where they live are a familiar sound of summer. In Illinois, they are more mysterious because their time here is fleeting. 

Common loons are only seen locally on their biannual migrations, first in March or early April on their way north and again in October on their return trip south, according to the Illinois Department of Natural Resources. Within the preserves, loons are occasionally spotted at a handful of preserves, including Whalon Lake in Bolingbrook, Rock Run Rookery in Joliet and Monee Reservoir in Monee.

The breeding territory for common loons includes most of Canada and Alaska and portions of Maine, New Hampshire, Vermont, New York, Michigan, Wisconsin, Minnesota, North Dakota, Montana, Idaho and Washington, according to the U.S. Geological Survey. 

The loons that pass through Will County on their way to their Great Lakes breeding grounds spend winter along the Gulf of Mexico and the Florida coast, according to the Cornell Lab of Ornithology. Other populations of common loons winter along the Atlantic and Pacific coasts.

Loons typically arrive in their northern breeding grounds in late April or early May, according to the U.S. Geological Survey. The Great Lakes population of loons is the largest in the country, with more than 10,000 adult loons in Minnesota and another 4,000 in Wisconsin. 

In their breeding plumage, common loons have a unique appearance that makes them hard to confuse with other waterfowl. Males and females have a similar appearance, with black heads, sharp black bills, red eyes, black and white checked backs and a prominent white band on their necks, according to the Loon Preservation Committee. Their winter plumage is much less striking, with a gray head, bill, neck and back and a white breast.

The common loon is one of five loons in North America, and although loons look like ducks, they are classified separately, the Loon Preservation Committee reports. Of the five loons in North America, the common loon is the most common and widespread and the only one that regularly passes through Illinois.

Like diving ducks, common loons dive completely underwater to hunt for food, mostly eating fish along with other aquatic invertebrates such as crayfish, according to the Cornell Lab of Ornithology. Unlike ducks and most other birds, however, loons have solid rather than hollow bones. This aids in their diving ability because they weigh more and are less buoyant. 

Loons only leave the water to mate and incubate their eggs, the Cornell Lab of Ornithology reports. They are good swimmers, but they move awkwardly on land because their legs are situated far back on their bodies.

Even in water, loons need a lot of space to take off, requiring between 30 yards and a quarter-mile depending on wind conditions, Cornell Lab reports. In addition, they can only take off from water. This can become a problem when they mistakenly land on wet roadways and paved surfaces thinking they are waterways. They can become stranded without water nearby for takeoff.

Like many other birds, loons are monogamous, with pairs typically staying together for about five years, Cornell Lab reports. If one half of the pair does not return to their breeding territory, the remaining loon will find a new mate. 

Both male and female loons participate in nest-building activities and share in incubating the eggs, which typically lasts about 27 days, the Loon Preservation Committee reports. Female loons usually lay between one and three eggs per breeding cycle. Parents take care of their hatchlings for about 12 weeks. During this time, it's not out of the ordinary to see the hatchlings hitching a ride on their parents' backs as they swim through the water.