Changing day length bird migration triggers bird migration because photoperiod is the only absolutely reliable environmental cue that signals birds that keeps seasonal time.
Ruby-throated hummingbirds have already begun to leave, and chimney swifts and nighthawks are gathering in evening skies as they prepare to head south.
Even more remarkable than bird migration, however, is the migration of monarch butterflies, lowly insects with a brain the size of a pinhead.
Monarch butterflies also respond to changing photoperiod and travel nearly 2,000 miles to the mountains of central Mexico each fall.
Monarchs east of the Rockies migrate to a few isolated patches of montane fir trees in central Mexico, while western monarchs winter along the southern California coast.
Mark and recapture studies have shown that eastern monarchs travel as far as 1,800 miles in just four months. They move only by day at a leisurely pace of five to 18 mph.
Much of what we know about monarch migration comes from a University of Kansas citizen science program called Monarch Watch (www.monarchwatch.com).
Members purchase self-adhesive tags to place of the hind wings of captured monarchs. During the winter, tags recovered in Mexico help biologists understand the complexities of monarch migration.
Year after year monarchs return to the same winter areas, even the same trees. So reliable are these migratory aggregations that they have become a major tourist attraction in Mexico.
In fact, ecotourism in Mexico may yet be what saves critical winter monarch habitat.
Until 1976, no one knew what happened to monarchs in the winter. (No one, that is, except the locals who lived in that part of Mexico.) Fred Urquhart (1912-2002), a Canadian biologist, devoted much of his career to solving this mystery.
He tagged his first monarch in 1937 and for nearly 20 years he, his wife and volunteers tagged thousands of monarchs with labels that read, “Send to Zoology University of Toronto, Canada.”
Finally, in January 1975, he heard from a couple who found millions of monarchs in a mountain in the state of Michoacan. The next year the Urquharts traveled to Mexico to see the spectacle themselves, and the August 1976 issue of National Geographic announced the news to the world.
The most remarkable aspect of monarch migration is that each butterfly makes the trip only once. Monarchs that leave the wintering grounds in late February or early March lay eggs on their way north, then die.
Consequently, several generations of monarchs separate those that return in the spring from those that head south in the fall. Yet somehow each fall, inexperienced monarchs return to their ancestors’ traditional wintering areas.
On the winter grounds monarchs are sluggish and inactive. They congregate on trunks of fir trees by the tens of millions. When February arrives, they still have plenty of stored energy for the trip north.
Mating occurs before migration begins, and females lay eggs on milkweed plants as they move northward. This insures a new generation of monarchs in northern Mexico and south Texas.
Subsequent generations lay eggs as they work their way north until monarchs return as far north as Canada.
The monarch migration follows the appearance of milkweed from south to north.
Though these burnt-orange and black insects are among the most beautiful butterflies, both the caterpillars and adults taste terrible to anything that eats them. In laboratory experiments blue jays vomit within minutes of eating a monarch because monarchs are what they eat.
Milkweed leaves are highly toxic to most animals. Though monarchs are unaffected by the toxin, they incorporate the poison into their own body tissues.
The poison is retained through metamorphosis so even adults have high concentrations of the foul tasting chemicals. Curiously, the toxin is more highly concentrated in the wings and exoskeleton than in the body. Thus, a predator that nips even a piece of wing learns that monarchs are distasteful.
As added protection, a monarch’s abdomen is tough and leathery, difficult for a predator to bite. These adaptations enable predators to learn that monarchs taste badly without necessarily killing the butterfly.
This also explains why we often see monarchs with badly battered wings — battle scars from the ongoing struggle between predator and prey.