Lately I have been thinking a lot about migration. At first blush you might think migration is a simple and basic process, so much so that most people take it for granted and don’t give migration a second thought. Birds leave in the fall and return in the summer. Enough said, right? But that is not what I think of when I think of migration. I think of the incredible rigors of migration. The days, weeks and sometimes months of endless flying, traveling in areas the birds haven’t been before on their way to a place they haven’t been before or know anything about. The challenge of finding food during migration in unfamiliar and often dangerous territory is also a daunting task. Trying to avoid a multitude of hazards along the migratory route is another problem. These hazards include obstacles such as power-lines, radio antennas, tall buildings, air planes, and the ever more common wind turbines. To me, it’s amazing that any bird is successful in migrating these days.

Migration is so difficult that of the species that migrate, only about 50 to 60 percent of the adult birds that are migrating will actually survive the journey. Compare that to non-migratory birds in the tropics which have a rate of 80-90 percent and you can see just how dangerous migration really is.

Let’s start with the definition of migration. Most would agree that migration is best described as the predictable or seasonal movement of a bird from one region to another for the purposes of surviving or propagation. The key here is the predictable or season movement. This means that each year around the same date the bird will start to migrate and they will end up in approximately the same place each year. And the reverse will be true in the opposing season.

Birds such as the American Goldfinch are really not true migrtors, but rather they just wonder around without a predictable destination or at a specific time. This is called nomadic. Some birds only move far enough to find food or open water such as waterfowl, these are considered partial migrators. Birds that travel hundreds or even thousands of miles to a specific destination on a regular time schedule is what I am most interested in. These are called true migrators.

 No one knows when or why birds started to migrate. There are several theories and of course we will never actually know. However we can be fairly sure migration started as a result of one or more of the following conditions; climatic change, lack of recourses such as food or water, competition for nesting sites, aggression or dominants of members of the same species, seasonal tracking of specific food such as fruit or nectar. No matter the reason, migration is a real natural phenomena.  

Here are some other things we know about migration. Bird populations can develop migratory behaviors and loose migratory behaviors fairly quickly. For example, when environmental changes occur that effects the reproductively of a bird species, migratory behaviors can develop dramatically fast; in just a few generations. House Finches are a good example of this. In their historic range these birds are sedentary or non-migratory. However in the early 1940’s House Finches were transported from California to Long Island, New York. The introduction was very successful and the House Finches colonized. However since the climatic conditions are very different from California to New York, the birds were under considerable environmental pressure to survive during winter. Within 20 years of the birds being introduced to New York, they started to migrate to get out of the winter. Although some of the birds did stay and remained sedentary many more started to migrate to the Gulf States and now they are considered a partial migrant.

There are also good examples of highly migratory birds that no longer migrate once environmental conditions changed. Dark-eyed Juncos colonized Guadeloupe Island, which is over 150 miles off the coast of Baja California. Since the climate conditions are fairly stable on the island the birds had no reason to migrate as they do in their normal range. Now the population of juncos on the island is sedentary.

So there you have it, a basic over view of migration. It’s best if we keep in mind that nature is dynamic and always changing. It has to be in order to adapt to changing environments and that is why Mother Nature is a survivor weather she is migrating or not. Until next time…