Bird’s Eye View on Migration

Tim Iverson

(6/2016) With the passing of seasons some birds stay while others go. By this time of the year most of our avian neighbors have relocated from their winter abodes to their summer homes amongst us. Migration is a well-observed event, but not always well understood.

The term migration is an umbrella term that encompasses several different variations on the bi-annual movement birds make. It’s important to note that some birds stay put all year long. Cardinals, for instance, don’t migrate and are yearlong residents finding suitable food and shelter no matter the weather. Some birds migrate very short distances. Short as in from the top of a mountain to the bottom of the same mountain. This type of migration is called altitudinal migration. Some of these species may spend their entire lives within a 20 miles radius. Medium distance migrants may travel the distance of a few states. Most impressive though are the long distance migrants. These are the birds that will travel thousands of miles crossing hemispheres to find residence in new quadrants of the globe for a few months at a time.

The longest migration belongs to the Arctic Tern, which breeds in the Arctic and winters in Antarctica, encompassing the globe for a whopping 25,000 mile round trip journey! One of our local long distance migrants who holds an impressive range is the Barn Swallow. In the summer Barn Swallows can be found as far north as Alaska and in winter as far south as Argentina and Chile. This bird prefers open spaces and can usually be seen swiftly skimming over fields. While historically building nests in caves, cliff sides, or natural overhangs they seem to have adapted to humans well and now usually construct nests in barns, bridges, or eaves. Barn Swallows make nests by clumping mud together in a cup shape and layering with grass and feathers.

Courtship often involves dazzling aerial acrobatics and chasing one another through the air. Once coupled a pair can be seen preening each other's feathers and touching beaks. A couple will produce 3 - 7 eggs at a time. Once hatched a Barn Swallow will be mature enough to set off on their own in approximately 15 - 28 days. A nesting pair can produce 1 to 2 broods per year. Barn Swallows can most often be seen sailing over fields hunting flies, beetles, grasshoppers, and other meadow insects. Once the summer months have peaked Barn Swallows will begin to make their way south. For them migration south is in full swing by late July, and will return north by May of the following year. Migration is a complicated process and not entirely understood by biologists and scientists.

Short distance migration is primarily driven by the lack of resources available during the winter months. Simply put, they’ll move to where survival is easiest. Long distance migration is less understood and not as easy to justify. There’s simply no need to travel thousands of miles for beetles and berries when there are other ones considerably closer. Research suggests these routes developed as a result of better breeding habitats. While tropical zones provide abundant food during the winter months, temperate zones are more habitable during breeding seasons. Longer daylight hours and ample food allow these migrants to produce twice as many offspring compared to their non-migrant cousins. Of course, during the tough winter months it’s time to head back to the tropics.

The switch that flips indicating when a bird should head south is initiated by a few factors. Shortening daylight hours, cooler temperatures, dwindling food supplies, and genetic factors are all cues that tell a bird when it’s time to go. Researchers have even documented that captive and caged birds exhibit restlessness during migratory periods and can be seen fluttering predominantly toward the north or south side of a cage depending on the season.

Migratory birds can cover thousands of miles during a single trip. They seem to intuitively know the way to go. Even first year birds who have never made the trip before can manage on their own. Biologists have wondered how that can be, and the best research indicates that they utilize landmarks, the sun and stars, and the Earth’s magnetic field. Many young birds are believed to develop a site attachment during the first migration and will continue to return to those sites year after year. Some birds, barn swallows included, may even use the very same nests time and time again.

Migrants often get bottle necked in what bird enthusiasts call migrant traps. These are areas that migrating birds concentrate into in very large numbers. These traps are often last pit stops, like islands or peninsulas, before long barren stretches of distance must be covered without readily available resources. Bird watchers can see many birds of different species in a short time span, which makes them exceptionally popular for people as well as birds. In some instances these traps might provide an important food sources for migrating species. The Red Knot is a long distance migrant that travels 9,000 miles from the coasts of South America in winter to breed in the Canadian arctic. While making this journey they make a pit stop in Delaware Bay. On some days it’s estimated that nearly 90% of the entire subspecies can be found there during these mass migrations. This trap is a midway pit stop on their long journey and provides an essential food source to power them through. They arrive in the spring when Horseshoe Crabs lay their green eggs in sand. The Red Knot devours as much as he can and continues his way onward.

Migration can be dangerous business. It is physically taxing and may be too much for older or injured birds to handle. It also increases exposure to predators. Man made structures like wind turbines also present problems. We place them where there is consistently the most wind so we can generate the most power. These sites are often where migratory birds make their routes because it is the path of least resistance for them. Climate change is eroding shorelines, altering food availability, and distorting weather patterns creating problems throughout the entire migratory cycle.

The phenomenon is as arduous as it is inspiring. The somewhat nomadic lifestyle of birds was shaped thousands of years ago and has also helped to ensure the survival of our modern day dinosaurs. Here’s hoping the prevailing winds provide smooth sailing for many more to come.