Each fall, some of our favorite backyard birds leave to go south for the winter because of autumn migration. After spending the summer in North America – during which they’ve bred a new generation – they begin to gather in groups and will soon leave their summer breeding grounds. They migrate — but why? We need to look far back in time to approach this important question.
Long ago, the earliest birds appeared on earth. They had an enormous advantage over other creatures — they had wings. When food was scarce, they could quickly travel to more abundant sources. As the earth changed and cooled, and seasons developed, birds had little trouble adapting. They could easily fly to warmer climates when necessary to survive. Perhaps this was the very beginning of the phenomenon we know as migration.
Some birds never migrate. They have adapted to the winter cold. Chickadees are an example. But for those that do migrate, such as hummingbirds, orioles, grosbeaks and many other birds, it is not a choice. Migration is a programmed response to environmental cues. Here in North America there are about 350 migrating species, 250 of which are considered Neo-tropical, meaning that, after breeding in the north where they summer with us, they fly south to spend their winter months in the warmth of Mexico, Central and South America as well as the West Indies.
It is generally accepted that the change in the length of the day causes a significant response in a migrating bird’s brain. Their appetite increases in order to build stores of fat to expend on the long journey, they become restless, and it’s believed they have an increased tendency to flock together as there’s safety in numbers as they travel on their journeys over thousands of miles.
In the US there are four major flyways, the Atlantic, Mississippi, Central and Pacific. These are very broad geographical areas of travel each containing hundreds of widely diverse migration routes. As the routes approach Central America they converge and then spread out again as each bird species returns to their own wintering grounds. Some birds end their journeys as close as Mexico, while others continue to Brazil and beyond. And almost unbelievably, the Ruby-throated hummingbird actually flies across the Gulf of Mexico, sometimes resting on drilling rigs or buoys on their way.
Resting places along migration routes are vital to traveling birds and they will use these known resting areas year after year. These locations provide food and water where the birds can restore their reserves before they continue on their migration route. Protecting these areas from development is extremely important for the success and survival of migrating birds.
How do birds know where they are going on these long journeys of thousands of miles? Birds use a variety of clues to help them on their way. They sight visual landmarks, follow rivers, coastlines and mountains. Day-migrants use the sun as a reference point, night migrants use the stars. And birds have a mineral called magnetite in their brains–thought to help monitor the earth’s magnetic fields to guide them north and south. And it’s probable that birds follow other flocks.
What is not known is how birds compensate for sunless days, interferences in magnetic fields or changes in landmarks. In the end, the phenomenon of bird migration still holds many mysteries that we have yet to discover.
– Written by Roxanne Brune