Objective: Students will learn about understand what migration is; its causes, length, mechanics, costs and benefits
Setting the stage: Ask the students if they’re comfortable. What would they do if they weren’t?
Chances are that they’d move to a better location. Similarly, animals move so they are in a more desirable location.
Migration: The periodic, large scale movement of populations of animals. Populations of birds, insects, mammals and fish all migrate.
Complete migration: All members of the population migrate. An example would be caribou in Alaska
Partial migration: Some animals migrate, some do not. An example would be the American Robin
Differential migration: Migration varies by class (e.g. age or sex) such as in the herring gull where young birds migrate a shorter distance than older birds and in the eastern United States female Dark-eyed Juncos migrate farther into the winter range than males.
Interruptive migration: Animals migrate some years but not others. Blue Jays exhibit this behavior.
The above definitions describe types of migration, however there is another way to differentiate types of migration into obligate and facultative migration and this describes why the species migrates.
|Obligate Migration||Facultative Migration|
Individuals in these species MUST migrate each year for survival. These migrations tend to be very consistent year to year in both their timing and their path. These are almost always complete migrations. The longest migrations are usually obligate migrations.
Individuals in these species “choose” to migrate or not. Their “choice” depends on resource availability. These migrations are usually done to find a great quantity of resources, even though resources in the current location have not run out. Partial migrations are typical in this case, and interruptive migrations are facultative as well. Facultative migrations are usually shorter in distance.
Not all migrations are the same. Grey Whales migrate between the warm waters of Mexico to the cold Arctic seas, while brown bats migrate only a very short distance. Certain animals take breaks along the way, while others travel nonstop. Hummingbirds bulk up before their big trek, or they may stop and eat along the way. Animals also differ greatly in the ways that they are able to navigate. Some animals, like homing pigeons, use their sense of smell, while others follow trails, use the Sun and stars, or follow coastlines. Yet others, like the arctic tern, feel the Earth’s magnetic pull. Many animals know where to go instinctively, while others (like Canada geese) have to be taught by their parents. There are many reasons why animals may determine that it’s time to migrate; they may be prompted by a change in temperature, in the length of daylight, or even in hormones that cause them to eat more and save fat for the journey.
It’s important for your students to realize that animals migrate for a reason. Kids can learn how to identify these reasons by comparing and contrasting the migrations of different animals. When your children grasp the various motives for animal migrations, they will gain a greater understanding about the natural world.
Why Animals Migrate:
For a wild animal trying to survive, long-distance travel is energy-expensive and dangerous! We would expect creatures who do not have to migrate to survive better than creatures who do. This has led scientists to conclude that migrations only evolve in a species if the benefits of the new location outweigh the high costs of getting there. The motivation for migration is different for different species. The three most common reasons for animal migration are:
1. …To move between sites that offer different necessary resources: sites for feeding, sites for breeding, sites for hibernation, etc.
2. …To avoid seasonal environmental conditions such as droughts, floods, or freezing temperatures.
3. …To follow the availability of their food source. This can tie in with seasonal conditions, as food sources (plants or prey) may be scarce seasonally.
Research to see if your location is in the migration path of any animals. What animals migrate through your area? Do any of these animals stop to feed or rest in your area? Make a migration calendar to show when animals will be migrating through your state or country. Then discuss some of the issues that migrating animals face in the modern world and have students propose solutions to these problems.
How they Navigate
How wild animals navigate during migration is a question that has fascinated humans for a long time! Some species are able to migrate vast distances (even across the world!) and they do not use a map, compass, or GPS device. In fact, some species are able to navigate through migration even though no living individual of that species has made the migration before! (Monarch butterflies are an excellent example of this). Clearly, the animals are navigating. But how? Scientists studying migration have found a number of different ways creatures navigate. Each species uses different skills and techniques to find their way, and most species are limited and cannot use all the methods listed below. However, most species have been found to have multiple navigation skills, so that they can use different clues as they get closer to their destination or as conditions change. For example, a species that primarily uses the sun‘s position to navigate might turn to using the magnetic field to navigate if the day is cloudy
5 Skills Used in Navigation
- Position of the Sun: Some species can determine the direction they are moving in by looking at the position of the sun in the sky.
- Earth’s Magnetic Field: Some species can sense the magnetic fields created by the Earth’s north and south poles and distorted by landforms. These animals use the magnetic fields almost as car lanes, knowing the direction they are traveling by their position in the magnetic field.
- Position of the Stars: Some species have been shown to use the stars and constellations to navigate. Experiments done inside planetariums have shown that changing the orientation of the starry sky will change the direction these species try to go.
- Smells: Some species use scent to recognize familiar places.
- Landmarks: Some species use visual clues such as rivers, mountains, or even smaller landmarks to find their way.
A number of terrestrial mammals migrate, most of which are large herbivores
One of the most impressive mammal migrations is “The Great Migration” and it isn’t a surprise that it is so named. Each year, in the Great Rift Valley of Tanzania and Kenya more than 1.5 million wildebeest travel across the grassy plains of the Serengeti to the northeast. These animals (also known as gnu) are members of the antelope family but look more like shaggy cows with big horns. They search for the grasses that flourish during the rainy season, traveling across hundreds of miles. Typically, zebra and other herbivores travel this migration route with them. This vast herd changes the behavior dynamics of all the animals in the area as predators attack the old, weak or ill members of the herd. Prides of lions, wild dogs, hyenas and even crocodiles all take advantage of this migration.
Another notable mammal migration is the caribou migration in Alaska. Caribou are large hoofed animals in the deer family and are the same species as reindeer, which are native to northern Europe and Asia. The differences between the two populations are that some reindeer have been domesticated as work animals while caribou are wild-living. Caribou are taller and lankier than reindeer. As summer approaches in the Arctic north, these herds may travel more than 600 miles to their summer grazing grounds. There the females, known as cows, give birth and feed on the rich grasses and plants growing on the tundra. As the days shorten and winter approaches with the first snow, the herd returns south, with females leaving several weeks before the males, to more sheltered areas where they survive the winter feeding on lichens.
In the lower 48 states of the US one of the more famous migrations is that of the elk in Yellowstone National Park. One of the largest deer species in the world, these mammals do not migrate great distances, rather they migrate significant elevation changes. The usual distance they travel may be 50 miles or so compared to the hundreds of miles of other deer species, however they can navigate as much as 3,000 feet of elevation change. As with the others mentioned here, the primary driver of their migration is food availability. In Yellowstone, tens of thousands of elk, in 7 or 8 populations, follow the greening of the summer grasses to high plateaus where they spend the summer having their calves and fattening themselves on the fresh vegetation. As their food begins to get covered with snow they start their trek back to the lower elevations. Since their migration is weather dependent, the timing has shifted over the past decades as the climate has changed. Studies have shown that elk have arrived in their winter ranges on average 50 days later in the season in 2015 compared to 2001. This change can have far reaching impacts on other inhabitants of the ecosystem such as predators and scavengers.