(This post was actually finished two weeks ago, but due to a failing home computer I didn’t save it like I should and I had to reconstruct the whole thing. Apologies for missing about a month-and-a-half.)
The study of geography talks about the reasons that people immigrate and emigrate (which by the way means to come into another country or leave one’s home country, respectively), the push and pull factors. These include:
- war (unfortunately all too present right now)
- running from the past
Animals have a more limited scope when they migrate. And the strict definition for migrate includes the idea of a return. Animals only permanently move out if habitat is destroyed or they are extirpated.
Here in Michigan we think of migration in primarily terms of north and south–Canada geese, mallard ducks, songbirds, sandhill cranes, and monarchs all head for sunnier climes beginning in September. But migration can happen in an east to west (or vice versa) cycle, up and down mountains, and even vertical movement through the water column.
All kinds of animals migrate (heck, even mold migrates!) from zooplankton to blue whales.
Migration for them is an “adaptive response to seasonal or geographic variation of resources.” Or sometimes to breed. The daylight length, temperature, or food sources have changed and so animals move on.
Those triggers–light, hormones, water currents–all give the signal to “pack up and head out.” Cooler temperatures signal birds to migrate, for instance.
So, how do they do it? Migrate, I mean. Animals use topography, polarized light patterns bouncing off of airborne particles (which happens even on cloudy days), the stars, and olfactory and magnetic cues. Rivers and coastlines are particularly helpful. The most southern point in Canada (well, nearby Pelee Island is officially the southernmost point) is a sandy peninsula known as Point Pelee. This helps guide birds across Lake Erie. Why wouldn’t it? It’s a giant directional signal pointing south.
Some animals forage along the way, while others have gorged themselves and built up fat stores (hyperphagia) much like those who stay and hibernate.
Southeast Michigan sees the beginning of migration in August and continues even to this present point as I witnessed a flock of Canada geese pass over me this morning. The little brown bats we observe wheeling the evening summer sky move from their tree roosts to caves for the winter. The trout and salmon are migrating for breeding purposes (though their numbers are limited in SE Michigan). Hummingbirds, sandhill cranes, and all kinds of warblers have exited the confines of the state. Out west, the elk migrate, and in northern Michigan there is a herd or four, but I don’t know that they migrate.
Before finishing this article let me include some random migration facts:
- Humpback whales complete the longest mammal migration of up to 8,500 km each way.
- Monarch butterflies, of course, have the longest insect migration from Canada and the northern US to California and Mexico in the fall–up to 4,750 km.
- Rattlesnakes(!) in Alberta, Canada migrate to find dens for hibernation just slightly south of their summer grounds
As annoying as the Canada goose has become, it’s still thrilling to hear the feathery thump when they fly in their characteristic V-shaped flocks closer to the earth on the way to wherever they stay. Occasionally, the mallards in smaller groups beat their whistling wings to find food further south. The great blue herons will stick around, surprisingly, as long as open water exists.
So what migrates from your neighborhood? Or perhaps you live in a winter destination? What “new” species are just showing up? Get outside and look around.