Migration

 

Animal migrations may become established in response to avoiding undesirable environmental conditions (deep snow, excessively hot or cold temperatures, drought, etc.), following forage quality and quantity availability through the year, or seeking seasonally available high-quality foods. Or, the drive to migrate and how to get to the appropriate location is so programmed into them that they have no choice in the matter.

Within some species, some populations are migratory but other populations are not. This is especially true for big game animals which have a wide geographic distribution, such as white-tailed deer. In this and many other northern areas, they migrate to and concentrate in locations where winter severity is less harsh. These areas are typically referred to as deer yards. Populations of white-tails in areas that do not face seasonally unsuitable conditions do not migrate.

Studies of where deer spend most of the year, but then migrate to a different area for the winter, sometimes show some interesting situations. For example, a radio-collared deer in the northern portion of the BWCAW in Cook County migrated west to a wintering area near Ely; another radio-collared deer in a similar portion of the BWCAW in Lake County migrated east and wintered near Gunflint Lake in Cook County. That may be interesting but not abnormal. Animals that care for their young for at least a year migrate as a family unit. Therefore, the young learn the migration route from their mothers and continue going to where their mothers took them their first year.

The above involve very short migrations; some are much longer. In Montana, many pronghorn antelope living south of the Missouri River migrate north 100, or more, miles and summer in Canada. A longer migration, but the young still learn the route from their mothers. In years of very high water, many (especially fawns) drown while crossing the Missouri River. Still longer migrations are common for whales which may migrate thousands of miles between breeding areas and prime feeding areas. Their calves also learn the migration from the mother.

However, bird migration is often much more complicated and interesting. As with mammals, some species migrate greater distances than do others; but none can be considered to be short. It is typical for local breeding populations to reassemble in a rather localized portion of their wintering area. For example, 6 sub-species of fox sparrows breed in localized areas along the west coast of North America; after migration, they are found in the reverse order to the south. The more northerly sub-species had “leapfrogged” over the more southerly sub-species. On the other hand, the arctic tern nests within 10 degrees of the North Pole and migrates to the Antarctic pack ice via the west coast of Africa.

Migrant birds (including inexperience juveniles) have the ability to fly on straight compass courses and know when and in which direction to begin their migration. This knowledge must be within the bird’s innate behavior. Migratory flights are not necessarily on a single compass bearing to their winter destination. Migrations of some species require making a change in direction part way through the flight; not necessarily related to topographic features the birds may recognize. These course changes and the migratory flights over vast areas necessitate some form of navigational ability.

Day time navigation involves the position of the sun, a sun-compass. Heavy overcast can cause some, but not total, disorientation as long as they can tell the approximate position of the sun. It is believed birds have some form of timekeeping to deal with the change in sun-angle through the day, a biological clock. Nighttime navigation is by the stars. Experiments with caged birds found good orientation only on clear nights. Overcast skies led to confusion as did twilight when the sun had set, and the stars were not yet bright. A study using bird-borne radio transmitters found straight nighttime flights could be made under overcast skies if there were clear or partly cloudy skies during the day or evening before the flight.

However, when flying over the sea and well away from land, birds fly a straight course both day and night; and do not appear to compensate for changes in wind direction. They may then drift off course and perhaps end up in the wrong area. There are some observations of these birds changing course at dawn to compensate for drift; and sometimes they fly into the wind to compensate for drift after they reach land.

In many species of birds, the adults and young do not migrate together. Typically, adults migrate a bit earlier than do the young. There have been studies in which young birds of a population were held until all others of their species had migrated; when released, all flew in the normal migration direction for their species. Other studies reared and released birds in areas where the species does not breed; they migrated in the direction normal for the population from which they had been taken. When they should leave and the appropriate direction that population should take was innately programmed into the birds.

Especially interesting studies involved capturing young birds of a population part way through their migration and transporting them well off to one side of their route; also, well outside of the area normally reached by birds passing through the capture location. In most cases the young birds continued to migrate from the release site in the direction they would have followed had they not been trapped. These birds followed a course parallel to the normal course and wintered in areas outside the usual range. The distance they traveled for the rest of the migration was the same as the distance they would have traveled had they not been displaced. They knew the direction to take and how far they still needed to travel but were not aware they had been displaced to the side. Therefore, they are innately capable of bearing and distance navigation. Correction for the displacement would have required a navigational ability they did not yet posses.

In a similar study, adult birds which had made the migration before could recognize they were not in the same location where they were trapped and had the navigational ability to set a new course to compensate for the displacement and arrive at the appropriate wintering area.

If juvenile migration seems remarkable, consider the monarch butterfly. These fragile insects migrate from northern and central U.S and southern Canada to central Mexico; western U.S. populations may winter along the coast of California. This is quite an undertaking since their flight speed is about 6 mph. The mature adults do not breed before they migrate; therefore, they can store additional nutrient reserves to help them survive the flight. In March, the overwintering monarchs breed and then start migrating north. They may make it to northern Mexico and southern U.S., but then they die. Successive generations of monarchs live for two to six weeks and continue migrating north. Eventually a generation reaches the summer destination. Every generation, after the one that started the migration, did what it was supposed to do in order to complete the migration that none of them had started.