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Elimination

When you get hot, your skin produces sweat. As the sweat evaporates, it cools your body. This is called evaporative cooling. Most mammals and birds use evaporative cooling to keep from overheating.

Mammals sweat. Birds lose water through their nose and mouth.

All animals produce nitrogenous wastes. These are toxic and must be eliminated. The kidneys are responsible for removing nitrogenous wastes from the blood.

In mammals, nitrogenous wastes are converted into an aqueous (soluble in water) substance called urea, which is excreted in the urine.

To save water, bird kidneys convert nitrogenous wastes into uric acid. White uric acid mixes with the feces before being excreted, giving bird scat (bodily waste) its characteristic color. Because they eliminate using uric acid instead of urea, birds require forty times less water than mammals require!

Birds must be as lightweight as possible to fly. Because waste (in the form of uric acid and feces) adds unnecessary weight, birds eliminate frequently. They normally eliminate just before taking off in order to be as light as possible.

Because their excretory systems are so effective, most birds are able to get all the water they need from their diet. This is particularly true for birds that feed on nectar, fruits, insects, or meats that contain a lot of water.

Black-capped Chickadee (Courtesy NEBRASKAland Magazine/NGPC)

This is a great adaptation because birds are vulnerable to predators when they visit streams or ponds to drink.

However, seed-eating birds such as the Northern Cardinal and the Black-capped Chickadee need to drink daily to replenish their body water. That's why it's always good to provide a safe water source as well as a food source for your backyard birds, especially if you live in semi-arid western Nebraska.

Even though Nebraska is far from the ocean, saltwater may be found in many lakes and ponds in the western Sandhills as well as some wetlands in the southeast part of the state. Some of these areas contain much higher concentrations of salt than the ocean.

American Avocet (Courtesy NEBRASKAland Magazine/NGPC)

This presents a particular challenge for birds. Bird kidneys are great at concentrating nitrogenous wastes, but they're poor at concentrating salts that must be eliminated from the body. That means birds must avoid drinking saltwater if at all possible.

American Avocets, for example, prefer to eat the crustaceans, insects, and plankton found in saltwater ponds. Using specially designed bills and tongues, they can filter the water from their prey before they swallow, so they get the food they need without actually drinking any saltwater.

Birds that must drink or feed from saltwater sources have salt glands. Salt glands rapidly excrete a salt solution when saltwater is consumed. The glands are located in special depressions in the skull, just above the eyes. When salt glands are active it can look like the birds are crying.