Why do birds do what they do?

Emily Renaud | October 30, 2018 | Newsletters

The powerful bill of the Northern Cardinal allows it to feed on rock-hard nuts and seeds, readily abundant during the winter months here at Manomet. Photo by Shiloh Schulte.

Emily Renaud

Communications Coordinator

Over the course of a year—or even a single month—we see a notable variety of bird species pass through the Northeast. In just September and October, you could potentially see hundreds of different species in Massachusetts alone; from shorebirds big and small in mid-September, to warblers and thrushes in early October, to sparrows later in the fall. What causes all of these different birds to move at different times in a single migration season? What are these birds doing? What do they eat? How do they do it?? The answer to these questions, to put it simply, comes down to biology.

Birds have a dazzling array of adaptations that allow them to live almost everywhere. Take, for example, the Chimney Swift. As aerial insectivores, swifts rely solely on flying insects as their food source. To accommodate this special diet, they must migrate to different geographic locations as insects become seasonally available. In addition to a limited diet, Chimney Swifts also have specific habitat needs; they use only hollow cavities, like you may find in an old tree, for shelter. As human populations increased, so did our urban settlement, and forested areas that once provided habitat for these birds were destroyed to make way for cities and buildings. Despite facing dramatic population declines due to major changes in habitat availability, the aptly-named Chimney Swift has adapted to using chimneys and other similarly-shaped manmade structures for shelter. Their unusually short legs, small feet and strong claws are perfectly suited to cling to stony surfaces. Now, you’ll often find large flocks of these birds flying into old chimneys at dusk (during late spring, when insects are most abundant here in Massachusetts) where they cling to the structure’s interior by the hundreds—sometimes thousands—to roost (or, rest) and nest.

Or, for a more seasonally-appropriate example, take a look at the Blackpoll Warbler. Blackpolls are a species of warbler named for the males’ black caps (or, “polls” if you’re old school) seen in their breeding plumage. Here at Manomet’s banding lab, we typically catch and band 200-300 of these birds every fall, usually from late-September to mid- to late-October. For Blackpolls—and several other migrant species—the timing of their travel is critical.

An average Blackpoll Warbler weighs around 12 grams—this individual was caught at the lab during a fall season, likely soon after arriving on the grounds for the first time after migrating from the north.

This Blackpoll, also caught during the fall, weighed around 20 grams—almost double its average weight!

Blackpoll Warblers are the longest distance migrant of any North American warbler. They spend their summers in the boreal forests of Canada, where they breed and nest from May through August. After nesting, they begin their over 2,000-mile southbound migration over the Atlantic Ocean to the rainforests of South America. Their first stop on this journey is often the forests along the Northeast and mid-Atlantic coasts. In order to survive their tremendous migration, Blackpolls must first double their body weight to sustain the amount of energy needed to complete their journey. Normally weighing 11-12 grams, these birds will stop to feed on the abundance of fall berries found on shrubs and trees in coastal forests like at our Plymouth, MA, headquarters. After a few days of gorging on berries along the coast, most individuals will depart on a nonstop, 70-hour flight south carrying an extra 10+ grams of fat.

With their pointed, slender bills, warblers are diet specialists, like the Chimney Swift, meaning what they eat is limited due to their morphology, or body structure. This, along with habitat availability and climatic variation, is one of the main reasons for their epic, biannual migration. Birds that move short distances seasonally, such as Northern Cardinals, Black-capped Chickadees, and Song Sparrows, are quite different. If you look at the bill of a chickadee and the bill of a warbler, you see the chickadee’s is stouter and thicker. This, as you may have guessed, allows for a more variable diet. It’s like when you think of mammals; carnivores have large, pointed teeth for shredding meat, while herbivores have generally dull but strong teeth, for grinding. Omnivores, though, have a combination of tooth types, and are thus able to eat a myriad of foods. The short, powerful bill of a chickadee allows the bird to crush seeds and nuts, an abundant food source in the winter, as well as nosh on the insects and berries more commonly available in the spring and fall. This generalist diet allows them to inhabit areas on a year-round basis, negating the need to migrate long distances for food.

Of course, there are other adaptations that allow birds to live their incredibly diverse lifestyles. Like, the long, slender legs of a Great Blue Heron which allow it to stride through deep water as it hunts for fish. Or, the razor-sharp talons of a Golden Eagle, giving it the power to tear through the flesh of small mammals. Or, the great, big eyes of a Long-eared Owl, positioned at the front of its wide, round face that allow it to see and hear its prey more efficiently as it hunts in the night.

These examples are only a small drop in the huge pool of bird biology; and, while we would love to continue here, we encourage you to learn more about these and other adaptations at our Basic Bird Biology class next month. We will take you through the basics of bird origins, structure and behavior with plenty of examples like the ones we’ve mentioned here. The class also takes advantage of Manomet’s robust teaching collection of specimens to help enrich the experience. From novice to pro, birders and bird enthusiasts of all levels will have something to learn from the final course in our Fall Birding Crash Course series for the year.

To register, visit our events page here or contact Evan Dalton, Lead Instructor, Landbird Conservation, for more info. We can’t wait to see you!