Mammals, including humans, inhale oxygen and exhale carbon dioxide, which sounds like a fairly simple process. But compared to birds, we’re very inefficient. When we breathe, we inhale oxygen-rich air into our lungs. This fresh air mixes with old oxygen-poor air already in the lungs. When we breathe out, we breathe out this old stale air along with fresh air. No fresh air is getting into our system when we exhale, however.
Birds do not breathe this way. When birds fly, their demand for oxygen is enormous. When they inhale and exhale, fresh air and stale air do not mix. The air moves in a one-way loop, and it takes two complete cycles of inhaling and exhaling to complete the process.
Here’s my crude rendering of how this works:
A bird inhales air into rear air sacs. When it exhales, this air goes from the rear air sacs into the lungs, pushing out the stale air already in the lungs into front air sacs. Then the bird inhales again, and the stale air in the lung is pushed into the front air sacs. This is the air that leaves when the bird exhales through its nostrils. Scientists think this adaptation developed when dinosaurs—ancestors to birds—lived in a low-oxygen environment and had to breathe efficiently to survive.
Like all birds, yellow-rumped warblers (Setophaga coronate) breathe in this fashion. They need to breathe efficiently because some migrate thousands of miles from winter grounds in Central America to Canada and Alaska where they breed. Now, mid-to-late spring, is the time of year they will pass through our region.
Most warblers have digestive systems that are designed to process soft-bodied insects. The yellow-rumped warbler, however, is an exception. Like other warblers, yellow-rumps eat mostly insects during breeding season in the summer. However, these birds regularly eat waxy fruits such as the berries of junipers, wax myrtles, bayberries, and poison ivy, in fall and winter. They especially like bayberries, which are covered in a solid, waxy layer that most birds cannot digest. But yellow-rumped warblers can digest 80 percent of bayberry wax.
Berries ripen in August through October and remain into the winter. They provide energy for birds that can digest them in northern and coastal regions of the United States. Where these berries abound—coastal New England and the mid-Atlantic—you might see yellow-rumped warblers lingering throughout the fall and even into the winter, when other warblers have already left for the tropics in search of food.
These tiny warblers have several adaptations that allow this to happen. First, material from the intestines flows backward into the gizzard, an extra stomach birds have that essentially chews their food, where it is ground a second time, allowing these tiny birds to further process waxy food. They also produce high levels of bile-salts, which are created in the liver, stored in the gallbladder and released into the intestines to help digest saturated fats. These salts act like soap to break down waxy layers. Once in the intestines, the food moves through very slowly, allowing the bird to process and absorb maximum nutrients from the berries.
Scientists think that these adaptations may have helped the species become one of the most widespread warblers in North America. As many as 170 million exist worldwide, from Alaska to Central America.
Even though these warblers are extremely common, we do not see them in the numbers that William Brewster, a late 19th and early 20th century ornithologist, described in “The Birds of the Cambridge Region of Massachusetts.” Back then they were also called Myrtle warblers because they ate wax myrtle berries. Here’s what he wrote:
[D]uring the forenoon of April 25, 1868, I found “several thousand” Myrtle Warblers at Fresh Pond. It was snowing heavily at the time, but on the north side of the pond, near the water’s edge, the snow melted as fast as it fell and the pebbly shores were bare. Here, for a distance of perhaps a quarter of a mile, the surface of the ground was literally crowded with Yellowrumps.
They were even around in the winter, according to Brewster:
On January 27, 1868, I saw a Myrtle Warbler…within what are now the grounds of the McLean Asylum, feeding on the berries of a greenbrier vine, … on a hillside not far from Arlington Heights a few birds have been found nearly every winter since 1890.
The same birds that once crowded Fresh Pond during Brewster’s snowstorm still visit our region today. Throughout May, look for these common migrants alighting in the canopies of trees. And delight in the knowledges that they have a respiratory system inherited from dinosaurs and a digestive system that can turn wax into food.
A stronger
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