Sep 20, 2017 | Featured in: Watching Backyard Birds, October 2017

The Complicated Process of Migration

American goldfinches swamp thistle sock feeders at a reader's backyard feeding station.
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Sometimes what you see is not what is really there.

Many folks will read that sentence, shrug, and assume, quite reasonably, that there has been a snag in the editing process or that the writer's cider has begun to turn.

Stay with me. Bird watching, like any specialized activity, has its own, sometimes confusing rules. How can it be true that what you see sometimes isn't what is actually there?

The answer is that migration, which appears to be a simple concept on the surface, is actually an immensely complicated process. Migration is widely understood to be the movement of birds from one place to another. We see migration every year in our yards: Sparrows and finches arrive from the North in the fall and leave in the spring. Geese fly overhead going north and south as the season dictates. So do hawks. Simple.

The big picture, however, obscures the complexity. Take, for example, a familiar species like the American goldfinch.

For many of you, the goldfinch is a daily visitor to the backyard chow line. Some readers have a few and some have small armies of goldfinches, but in almost every part of the continent goldfinches are regular visitors, even in the summer.

Let's assume you have a flock of about a dozen. Each day they are there, cheerfully elbowing each other for the best seat at the table. They are entertaining and reliable. Because the size of the flock does not seem to change much from day to day, it is natural to assume that you have acquired the affection of the local flock. Remember, however, that what you see may not be what is actually there.

Even if you have exactly 12 goldfinches come to the feeder every single day for a month, it is almost certain that you aren't seeing the same birds at the end of the month that you were seeing at the beginning. Hidden by the regularity of their appearance can be a fairly considerable migration. What is going on?

Goldfinches can be both facultative and leapfrog migrants. (The more scientists learn about migration, the faster they have to invent words to describe it. There are dozens of "kinds" of migration, including not only facultative and leapfrog, but upslope, downslope, differential, partial, irruptive, trans-equatorial, etc.)

Back to our facultative, leapfrogging, goldfinches. A facultative migrant is one that moves in response to local conditions. Facultative migrants have thrown off the limitations of many birds, the ones programmed to fly from point A to point B and back every year. Facultative migrants assess the weather, the local food supply, and other factors and either stay put or hit the road depending on what they find. They may start at point A (and stay there), or go to point B and then point C and then point D. They are not looking for a specific place, they are looking for the right circumstances.

There are numerous birds that are facultative migrants. Depending on which part of the continent you are living in they can include song (and other) sparrows, waxwings, finches, flickers, and waterfowl.

But birds are not always just one kind of migrant. Goldfinches and many others are also leapfrog migrants. That means that more northerly birds always head south, no matter how sedentary the more southerly populations are. When the northern visitors start arriving in the south they find much of the local habitat already occupied, so they "leapfrog" over the residents and go farther south. And sometimes the southern birds are partial migrants, which means that the adults, or the males, or the really, really stubborn ones stay put, and the others wander off, to be replaced, at least for the winter, with birds from the North.

The closer you look, the messier it gets.

The reality, as opposed to the theory, of migration, came to me while I was helping a friend band birds at his feeder. Every day about 30 house finches showed up and we dutifully caught and banded many of them. And every day we caught birds without bands. Lots of them. The flock always seemed to be about the same size, but after a month we had banded hundreds and hundreds of house finches. Unless the birds were deciding the bands didn't go well with what they were wearing and they were taking them off, we had a river of finches moving through the yard. What we thought we were seeing, which was a resident flock of finches, is not what was actually there.

This kind of migration is hard to see if you are not banding the birds. There are hints though. All house finches do not look exactly alike, although most are so similar that most humans can't be sure if they are seeing the same bird or different ones. A few really stand out: sometimes they have a white patch on the wing, or an unexpected spot on the head, or a missing tail feather. Often we overlook such subtle clues. As the season wears on, a flock that seemed to be mostly females and young birds may slowly change into a flock that is mostly adult males. When that happens you can be fairly confident that migration is happening. Precisely what type of migration we'll leave to the scientists to figure out.



About Eirik A.T. Blom

Eirik Blom was a contributing editor for Bird Watcher's Digest from 1998 to 2002.

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