Monarch butterflies
Monarch butterflies cover every inch of a tree in Sierra Chincua, Mexico (where the insects spend winters), as captured by National Geographic photographer Joel Sartore.
The monarch population has gone from an estimated billion butterflies in the mid-1990s to only 35 million today, according to the Center for Biological Diversity - a decline of more than 90 percent.
By CBSNews.com senior producer David Morgan
Take Off
Monarch butterflies in the Sierra Chincua sanctuary.
Perched
Monarchs spend the winter in Mexico; then in March, they begin their yearly migration northward. They glide right up the center of the North American continent, where they need plenty of nectar-bearing plants to feed on, and especially milkweed to lay their eggs.
Milkweed Seeds
Milkweed seeds.
Most monarchs are born in the Midwest's Corn Belt, where its favored plant, milkweed, is disappearing, due to insecticide spraying of corn and soy. Climate change is also throwing predictable flowering seasons for the native plants on which the monarchs feed in flux.
Milkweed Beetles
Milkweed beetles on milkweed plants at Ryerson Woods in Deerfield.
A single female monarch can lay hundreds of eggs on the leaves of milkweed - the only food source for the insect in its larval stage.
Caterpillar
A monarch butterfly caterpillar at the Butterfly Pavilion of the Lincoln Children's Zoo.
Caterpillar
A monarch butterfly caterpillar (Danaeus plexippus) climbs on cudweed at Spring Creek Prairie.
Caterpillar
A monarch butterfly caterpillar (Danaeus plexippus) climbs on cudweed.
Perched
Each adult monarch lives about four to five weeks, which means the adults who begin the migration from Mexico lay eggs in the Southern U.S. and die, only to have the subsequent generation of butterflies continue the migration north.
Portrait
Even though it may take several generations to complete one migration cycle, monarchs return to the same Mexican forests where their ancestors originated.
Sierra Chincua
Monarch butterflies cover a tree branch in Sierra Chincua.
According to the World Wildlife Fund, monarch butterflies travel between 1,200 and 2,800 miles, or more, from the U.S. and Canada to forests in central Mexico to hibernate.
Sierra Chincua
Monarch butterflies cover every inch of a tree in Sierra Chincua in central Mexico. The ancient fir trees are so laden with monarch butterflies their bows literally bend under the weight.
Sierra Chincua
Monarch butterflies cover every inch of a tree in Sierra Chincua.
Sierra Chincua
Monarch butterflies cover every inch of a tree in Sierra Chincua.
Sierra Chincua
Monarch butterflies cover every inch of a tree in Sierra Chincua.
Sierra Chincua
A winter sanctuary for monarch butterflies, in Sierra Chincua, Mexico.
Migration
Monarch butterflies in flight in the Sierra Chincua sanctuary.
Spread Your Wings
Male monarchs have a black spot (a scent gland) at the center of each hind wing, as seen here; females do not.
Butterfly Pavilion
A monarch butterfly at the Butterfly Pavilion at the Lincoln Children's Zoo.
Hello!
The Sartore family makes friends at the Butterfly Pavilion of the Lincoln Children's Zoo.
Closeup
A monarch butterfly at the Lincoln Children's Zoo.
Wings
The Center for Food Safety, in a report released in February 2015, states that over the past 20 years, glyphosate, a milkweed-killing herbicide used with genetically-modified corn and soy crops, has virtually eradicated milkweed from Midwestern states that make up the monarch's primary breeding ground.
Dining Out
A monarch butterfly feeds on Lantana camara.
Increasing the proliferation of flowering plants on which migrating monarchs can feed - purple coneflower and asters, vervain, black-eyed Susans, and above all, milkweed - would help boost their numbers.
Couple
Danaus plexippus.
Flurry of Activity
In December 2014 the U.S. federal government announced it would consider listing monarch butterflies under the Endangered Species Act, to help protect their habitat.
Up Close and Personal
A photographer in the Sierra Chincua monarch sanctuary.
For more info:
Follow @JoelSartore on Twitter and Facebook
Instructions on growing your own milkweed (Monarch Watch)
The Plight of Monarch Butterflies (Environmental Defense Fund)
Monarch Butterfly (National Wildlife Federation)
Monarch Butterflies Fact Sheet (Defenders of Wildlife)