During the last few days, I feel like I can notice fall slowly approaching. The decreasing day length and dropping temperatures are a signal to birds (and even some humans!) to start preparing to head south. Despite the misconception that birds don’t like the cold, many birds can survive freezing temperatures, so it’s not necessarily the cold that drives them away. It’s our waning resources. It’s hard to find food – whether that be seeds, worms, mice or something else – when everything is covered in snow. Migration can be defined as a movement from areas containing few resources to areas containing abundant resources. In the spring birds head north in a hurry, following the emergence of insects, prey, and plants that are important to their diet. The Northwoods provides an abundance of insects throughout the summer, which is a crucial source of protein for nestlings. Now, as fall slowly approaches with winter right behind it, many of these resources are beginning to disappear and our local birds have one foot out the door. But how can they fly hundreds, sometimes thousands, of miles? How can they sustain themselves? Their flight? How do they even know where to go? While many aspects of migration still mystify scientists around the world, light has been shed on some of the secrets of migration.
Birds were born to fly. Not groundbreaking news? But really, they are fine-tuned flying machines. Many people are aware of the importance feathers and hollow bones play in the role of flight, but it goes much deeper than that, quite literally. Nearly all of the organ systems in a bird have been modified to aid these creatures in their remarkable mode of locomotion. Their skeletal and muscle systems provide the physical requirements for flight. They have pneumatic bones, meaning there are air spaces in between crisscrossing struts, which allow the bones to be strong, yet light. Many of their bones are fused together, including all of the vertebrae (except for the ones in their neck) providing the rigidity required for flight. The sternum has a large, thin keel (below in blue) protruding from it, serving as an attachment area for the massive muscles involved in flying. The location of the keel helps keep the main mass low in the body, increasing aerodynamic stability during air time.
Even their digestive systems are designed to process an energy rich diet quickly and efficiently. Birds need to eat a lot to keep up with the high energy demands of flight, but they don’t want to be carrying around that extra weight as food moves through the digestive system – so it does so quite rapidly! A shrike can fully digest a mouse in three hours; a thrush can digest a meal of berries in just 30 minutes! The circulatory system has to work fast enough to support this high metabolism. Birds have a four-chambered heart, like mammals, but it’s quite large and strong, comparatively. It also beats extremely fast (a chickadee’s heart beats 500 times per minute at rest, increasing to over 1000 beats per minute while active!) to create a high pressure system capable of keeping up with the high metabolic rates required for flight. How can their bodies keep up with the oxygen demand? Birds have very unique lungs that utilize a series of air sacs for storage that essentially provide the bird with an almost continuous stream of oxygenated air. They also have a very efficient, vascularized system for getting oxygen to the muscles to sustain flight. Even their excretory system is modified to fly! Birds excrete waste in the form of uric acid, which has a very low solubility compared to the urea of mammals. This means they have a much less of a need for water and therefore carry less water weight. Anything that reduces weight is going to make it easier to fly! Lastly, birds have the most developed sense of sight in the animal kingdom – not a bad thing to have if your transportation method is head-first, high velocity flight. Incredible, right?
It gets better. Their navigational systems give their anatomical systems a run for their money. Bird migration is highly varied – some travel thousands of miles, some travel to the state next door; some migrate in one long continuous flight, others travel more leisurely, stopping many times to refuel; some go one way north, then take a different route back south. But no matter how they choose to migrate, they have to know where to migrate to. Different birds use different methods and many likely use a combination of more than one method. Some birds navigate by visuals – they follow topographical landmarks such as rivers, mountains, lakes, etc. Some birds have a magnetic compass that uses the earth’s magnetic fields to align them in the correct direction for migration. Other birds have an incredible “internal clock” that keeps track of time extremely accurately. They can then use this clock in collaboration with the sun’s movements to figure out directions. But not all birds migrate during the day when they can use the sun. That’s okay, because birds have a solution to that, too. Experiments conducted in a planetarium proved that some birds use the stars as a map, orientating themselves using the location of the constellations that rotate around the North Star! However they complete this magnificent feat, it truly is quite astonishing. The Arctic Tern, the world’s record holder for migration distance, navigates north past the Arctic Circle to reach is summer breeding grounds and then south to Antarctica for it’s wintering grounds – a distance of nearly 12,000 miles! These birds can live to be over 30 years old and the distance they fly in their lifetime is the equivalent of flying to the moon and back… and then there and back again… and then once more.
So it’s time to get outside and observe some of these awe-inspiring birds as they pass through briefly! Unlike in the spring, when birds are singing to attract mates for the breeding season, our fall migrants tend to be fairly quiet. Many species are already on the move, like warblers, great blue herons, nighthawks, swallows, martins, and orioles. In September, we’ll see migration kick up a notch with geese, sparrows, flickers, kingfishers, robins, juncos, grosbeaks, and hummingbirds. By October, many of the lingering water birds will also depart, including mergansers, ducks, loons, and swans. If you want to help, make sure your feeders are full this month as birds need a high caloric diet in preparation for their journeys!
So get outside and watch the action! If you’re looking for an exciting adventure, take a day trip up to Duluth’s Hawk Ridge. This spot is along the primary Midwestern migration route for many species of hawks. If you go on a September day with winds heading south, you are bound to see many – observers frequently count thousands in a day! Even if you don’t see any, you sure can’t beat the view. While you’re there, there are many hiking trails to explore where you can see many migrating species besides hawks. Or check out one of the many other Important Birding Areas (IBAs) in Minnesota! Even if you just head out around home, there is a lot to see this month! So get outside and enjoy some brisk hikes in the beautiful scenery of fall!