26.4: Migration

Migration is long-range, seasonal movement from one region or habitat to another. This common strategy, carried out by many different organisms around the world, is an adaptive response that typically corresponds to changes in an organism’s environment, like resource availability or climate. Migrations can involve huge groups of thousands of animals as well as single individuals traveling alone and can range from thousands of kilometers to just a few hundred meters.

Why Animals Migrate

For many migratory species, food resources are a major driving force behind the migratory movement. The Mexican long-nosed bat is a nectarivore that feeds on the flowers of plants (including agave) and undertakes seasonal migration tied to food availability, which varies seasonally.

Outside of the relatively stable climates of the tropics and subtropics, resource-based migration may also be tightly linked to climate. For example, beginning in early fall, Monarch butterflies migrate from Canada and the northern United States to the forests of Mexico to overwinter. This is linked to the seasonal availability of their host plant, milkweed, but also intrinsically to climate, as the butterflies would be unable to survive in the low temperatures of winter in higher latitudes.

Additionally, animals may undertake migrations to breed or produce young. Adult Atlantic horseshoe crabs inhabit the East Coast of the United States, and migrate to shallower waters each spring to mate and lay eggs in protected sandy shores and bays. Salmon also famously undertake lengthy and dangerous migrations to reach their spawning grounds.

How Animals Migrate

Migration can be obligate or facultative. In obligate migrations, individuals must migrate. In facultative migrations, individuals can choose to migrate. Obligate migrations are often complete migrations, in which all of the individuals in the population participate. However, migrations can also be partial, with only a fraction of the population migrating. Some individuals or groups within a population may also migrate farther than others, which is known as differential migration. For example, dark-eyed juncos migrate different distances into the overwintering grounds based on sex, with females tending to travel farther south than males.

Timing and cues that animals use for migration vary widely but can include factors like day length (photoperiod), resource levels, or temperature. Animals that migrate can also navigate in different ways, potentially using geographical, chemical, or even magnetic cues. Pigeons, for example, use magnetoreception to navigate.

Finally, some species undertake migrations in large or small groups, like geese, whereas others may migrate alone, like the Swainson’s Thrush. In some instances, animals complete their migrations over multiple generations, so that no single individual makes the entire trip. For example, the complete migration cycle of monarch butterflies takes around four generations.

Migration is an adaptive response that corresponds with changes in an organism's resources.

For instance, during a prolonged dry season when water becomes scarce for several months, African elephants must roam long distances in search of the massive amounts of food and water they need to survive.

Depending on the environmental circumstances, family groups often separate from the much larger herd to efficiently navigate the limited supplies along the route.

In this case, a dominate female leads at the front line, and another one guards at the rear of the pack to protect and supervise their young.

When the rainy season returns, the elephants can revert to their original regions to graze on the now lush vegetation that the rain regenerated.