- Living on the Wind: Across the Hemisphere with Migratory Birds
We get a lot of questions about migration. Do the Decorah eagles migrate? Do our Peregrine falcons migrate? Where do they go when they leave? We fitted our eagles with tracking devices in part to help answer some of these questions. To date, we've learned D1's migratory paths and summer and winter ranges, observed an interesting difference in dispersal patterns between D1 and D14, and an interesting similarity between D14 and Four.
I used to think migration was very simple. Like a lot of people, I thought that all birds except chickadees, pigeons, crows, and woodpeckers migrated once it got cold. They went south to escape snow and ice, returning to nest when the weather warmed up. 'South' was anywhere it didn't snow, or at least didn't snow much: Georgia, the Gulf Coast, South America. I had no idea that many birds don't migrate, or that 'south' could be Minnesota or Wisconsin. Peregrines, Snowy Owls, and Bald eagles taught me otherwise.
Among birds, migration is the regular, endogenously controlled, seasonal movement of birds between breeding and non-breeding areas (Salewski and Bruderer 2007). Bald eagles and Peregrine falcons are partial migrators - that is, some members of the species migrate and others don't. This is the most common type of migration, which makes sense since migration is driven by a number of factors, including daylight length, food availability, weather, the time it takes to raise young, and the distance between wintering and breeding grounds. Migration allows exploitation of different habitats as environments change seasonally or successionally (Dingle, 1996). Food availability seems to play a very important role in the migration of Bald eagles: inland northern Bald eagles tend to move southward after ice and snow start putting a lid over their favorite food source - fish, while southern Bald eagles are thought to move northward once warm weather drives fish into deeper water (although there is some debate about this). Weather can also impact migration timing in other ways: for example, a favorable wind pattern might help compel a bird to leave for its wintering or summering grounds if other factors are in place.
|D1's looping migratory path in 2012 and 2013|
So how do birds navigate? Migration studies have found five major methods:
- Magnetic sensing: Some birds, including pigeons, are able to use the direction and strength of Earth's magnetic fields to orient themselves. http://www.nytimes.com/2012/04/27/science/study-sheds-light-on-how-pigeons-navigate-by-magnetic-field.html?_r=0
- Geographic mapping: When I'm in Minneapolis, I use a number of tall buildings to help me orient the city. It turns out that birds do the same thing, using landforms and geographic features such as rivers, coastlines, and mountain ranges to guide them.
- Celestial navigation: migratory birds use the position of the sun (during the day) or the rotation of stars (at night) to orient themselves. Experiments done by Dr. Emlen in 1967 indicate that celestial navigation is learned. http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
- Wind patterns: A recent study at Cornell using crowdsourced data found that passerine birds are heavily dependent on wind direction for migration. By shifting routes, birds take advantage of stronger tailwinds in spring and less severe headwinds in fall.
- Learned Routes: Some bird species, such as sandhill cranes and snow geese, learn migration routes from their parents and other adult birds in the flock. Once learned, younger birds can travel the route successfully themselves.
In addition to watching our eagles, keep an eye on the animals outside your window. We know fall is here, and so do they.
- Birds may begin gathering in huge premigratory flocks, gorging on food to build fat reserves, and exhibiting restlessness, also known by the marvelous word zugunruhe. These behaviors help prepare birds for migration. Keeping a well-stocked bird feeder helps them too.
- Juvenile peregrine falcons are on the move! You'll sometimes catch a stranger on cam as they disperse down river. Please let us know if you get any band numbers!
- Frogs and turtles may begin migrating from summering breeding grounds to deeper bodies of water. While their travels are short, they still fit the definition of migration. For more on frog hibernation, click here. The Minnesota DNR has a very good article on helping migrating turtles cross roads, some cities close roads, and volunteers help keep migrating frogs safe.
- Some insects migrate and others hibernate. Aggregating paper wasps may be getting ready to hibernate, while dragonflies migrate in large numbers (or check this source out).
- Watch out for migrating Mule deer! According to National Geographic, they make a 150-mile migration twice yearly from the low desert to the high mountains.
Have a happy fall and take the time to watch for migrating birds and other animals! Would you like to know if the winds are favorable for bird migration in your area? Check these links out:
- Wind Map: http://hint.fm/wind/
- Intellicast Wind Map: http://www.intellicast.com/global/wind/current.aspx?region=namer
Some things that helped me write this post:
- Migrating raptors of the world: their ecology and conservation. Keith L. Bildstein
- Living on the Wind: Across the Hemisphere with Migratory Birds. Scott Weidensaul
- All About Birds: Navigation -http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
- Weather patterns aid small birds:
- Lissajous patterns. It struck me that the eagles' loop travels somewhat resembled a Lissajous pattern. A little googling found this very cool project: http://kaurov.com/wordpress/?p=692