Monday, November 3, 2014

The Mystery of Migration

It’s officially autumn. Autumn means cool weather, hot cider, and many of us wishing we could make like geese and head south for the winter.

All we’d have to do is look at a road map, choose a destination and away we’d go. Geese, of course, not being able to read, don’t have the luxury of a map. So how do they know where to go? Furthermore, how do migratory birds know when to leave?

Sandhill Cranes in flight (photo: Wikipedia files)

The Cornell Lab of Ornithology estimates that there are over 650 species of birds in North America, and most of those are migratory, meaning that entire populations of birds are moving to a different place. Many of us assume that migration must mean long distances, crossing at least half of the country. The term migration, however, doesn’t only encompass such epic journeys; it also includes smaller ones. A species that simply moves from a higher elevation to a lower elevation is considered to have migrated. Birds that migrate such short distances might not have trouble finding their destination since it is relatively close to their starting point, but what about the birds that migrate medium to long distances?

It is not well known exactly how birds navigate from one location to another during migrations. Even first year birds which have never experienced migration can navigate following a precise pathway to the exact same location as other birds without the guidance of an older bird. Some birds appear to navigate by sensing the earth’s magnetic field, others seem to migrate using the stars for navigation, and some use a combination of techniques.
Wading birds in flight (photo: Wikipedia files)

Furthermore, the triggers for migration are not well known, and also likely vary from species to species. Some birds may begin migration as temperatures cool or as days shorten. Other species may only migrate when food sources become scarce. There may even be a genetic predisposition in some species, since certain species have a very precise window of migration, even as short as two weeks, while other species migrate over a period of a few months.

More is known about the reasons why birds migrate than how they migrate. While it’s certainly nice to escape cold weather, the primary reasons for migration are food and nesting locations. Northward migration in the spring brings the birds to rising populations of insects as well as new nesting locations, giving the earliest arrivals the pick of the best nesting spots. Southward migration in the fall brings the birds to fresher, larger food sources.

While the knowledge that birds migrate is common, surprisingly little is known about this phenomenon. The World Bird Sanctuary’s bird banding team does their part to better understand the mysteries of migration during their spring migration banding blitz, when they catch and release songbirds passing through the area.  The banding team also bands songbirds as they migrate south, and they have recently started banding Saw-whet Owls as they migrate south.

Submitted by JoHanna Burton, World Bird Sanctuary Naturalist

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