When a loon is several years old, his readiness for mating shows in fancy, striped and patterned plumage. He sets up a territory on the edge of a lake and attracts a female by making yodeling sounds. Once a female responds, the male and female court each other by mewing softly and singing duets. If all goes well, they become a mated pair. The mated pair builds a nest of aquatic plants and mud on the water's edge. They defend their territory from other loons using a variety of aggressive calls (including toots, wails, and tremolos). The female typically lays two eggs, and both parents incubate them, taking turns. The chicks that hatch after about a month ride safely on their parents' backs around the lake. Until they mature, they peep and yelp for food and are rewarded with small fish, insects, and crayfish that the parents catch.

All modern birds have bills and no teeth. The shape of a bird's bill says a lot about what it eats, for example whether it specializes in seeds (stout , cracking bill), fish (pointy, spearing bill), or plants (wide, serrated bill). Birds swallow their food without chewing, so it travels to the stomach whole or in large pieces. Bird digestive tracts have some special features for digesting chunky food. A pouch in their throat (the crop), is used to store food to be digested later, or regurgitated to feed the young. An extra, muscular stomach (the gizzard) grinds food up. Birds are endotherms, using heat they make internally to keep warm. While a few species allow their body temperature to drop at night (torpor), a nearly constant body temperature is maintained by most birds most of the time. Continuously making heat requires fuel to burn, in the form of food. So, birds spend a lot of time eating.