Dark-eyed Junco (junco hyemalis) in my garden.

Animal Migration

Dark-eyed Junco (junco hyemalis) in my garden.
Dark-eyed Junco (junco hyemalis) in my garden.

Fall migration has begun and the animals are on the move.

What’s Behind Animal Migration?
Migration is the mass movement of large groups of animals or people from one place to another. It is often because conditions have changed so that staying where they were will make life difficult.

Hibernation is staying in the same place and sleeping through the worst conditions. Many small-bodied mammals tend to deal with the seasonal changes by eating so much that they put on a layer of fat and by hibernating in the winter.

Aside from certain tropical and evergreen forest areas where migrations are relatively uncommon, a large number of both aquatic and terrestrial species move from one habitat to another at some time during their lives.

Why Do They Move?
I think the reasons behind the moves are pretty obvious. The changing temperatures seem to be the biggest factor causing animals to move.

Here in the north it gets cold. Autumn and winter bring heavy rains, snow and ice storms. The plants and insects that birds, butterflies and other animals depend on for food, disappear.

These food sources go dormant or die with dropping temperatures. Another reason for mass movement is reproduction.

Animals, particularly birds, move to areas that are very favorable for raising young. Birds migrate north in the spring to take advantage of the emergence and abundance of insects and other foods.

The same individuals reappear every year in an area. They often arrive within a few days of a certain date each year. An example would be the cliff swallows of Capistrano which return to the town every year around March 19th.

Human often migrate due to war famine, disaster, disease and bad living conditions.

Many factors are thought to trigger mass movement. Most fall into one or two categories.

  1. Programmed events at predictable intervals in lifestyle or life cycle.
  2. Seasonal variations – change in day length, temperature, or abundance of food, episodic, density population movements, over large populations beyond environment carrying capacity

Who Moves?
Many animals move. Some travel thousands of miles and some just dig a few feet deeper into the mud of a pond, lake or stream. The most common travelers are birds. Birds like insect-eating songbirds, artic terns, Canada geese and many others move from one area to another as the weather takes a turn for the worse and food becomes scarce.

Who else moves?
Monarch butterflies, wapiti, caribou and many fish also migrate. Some high mountain dwelling animals move from high mountain tops to the lower mountain valleys. They move from high mountain elevations to lower elevations to escape severe winter storms. The heavy snow and ice makes finding food difficult. The wapiti of Yellowstone Park move to the valleys of the Rocky Mountains, and caribou move from the tundra to the coniferous forests.

Animals That Move

  • Atlantic SalmonSockeye Salmon
  • Monarch butterfly
  • Painted Lady butterfly
  • Broad tail Hawks,
  • Red Wing Blackbirds
  • Canada Geese
  • Wapiti (elk)
  • Caribou
  • Moose
  • Birds
  • Lobsters
  • Bees
  • Tortoises
  • Bats
  • Fish, brittle starfishes
  • Newts and toads
  • Insects

How Do They Navigate?
Animals seem to use several ways to find their way as the move from one place to another. Animals use a variety of environmental information to locate their positions and appropriate travel paths.

Most species have been found to use more than one type of information to navigate. Young individuals may also learn from older more experienced adults.

Ways by Which Animals Find Their Way

  • star navigation
  • the position of the sun, moon and stars
  • gravitational field of the earth
  • magnetic fields
  • ultraviolet light (on overcast days)
  • polarized light (on overcast days)
  • environmental odors (like the smell of the ocean)
  • coastline and shoreline configurations
  • water currents
  • visual landmarks

Here in Philadelphia we will be watching the migration of songbirds and birds of prey in the coming weeks. And the migration of Monarch butterflies along the New Jersey shore.

Those dead animals we see now along the roads are migrating and not making it across the road. I have seen dead opossum and skunks in the last week or two. The dead animals tell us who lives near us, sometimes without us knowing it.

Sources Consulted“Migration”, by Sneed B. Collard in Magill’s Encyclopedia of Science: Animal Life, vol. 3 Lemurs– Respiration, , p. 1036 – 1041. “Migration” by Valerius Geist, PhD in Grzimek’s Animal LifeEncyclopedia. Vol. 12: MammalsI. 2nd ed. Detroit: Gale, 2004. 164-170. 17 vols.

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