The marvelous spectacle of spring migration is here. Soon, the Black-billed Cuckoos, Blackpoll Warblers and Yellow-bellied Flycatchers will arrive in good numbers. The appearance of these late-arriving species signals the end of the spring migration.

Today, I want to consider why birds migrate. Each spring, several billion birds stream into North America from Caribbean islands, Central America and South America, where they have spent the winter. Why do so many birds undertake these arduous migrations? The benefits must exceed the costs. What are the benefits migrating breeders receive?

To answer these questions, we have to understand both why birds migrate north to breed and why they leave their northern breeding grounds to return to tropical or subtropical areas. The explanations stem from the fact that the earth is tilted on its axis.

Let's start with equatorial regions. Despite the tilt of the earth, equatorial regions have 12 hours of daylight each day. As one proceeds north, the length of day and night becomes unequal. When the northern hemisphere is tilted toward the sun, we have summer and temperate regions have day lengths that increase as one moves toward the North Pole.

The long summer days mean that plants can photosynthesize for sixteen hours or more each day, longer than plants at the equator. So during the northern summer, plant production spikes, providing food for insects that in turn provides food for birds. During the northern summer, food availability in temperate regions may exceed that of tropical regions.

Birds migrate to the north to take advantage of the summer flush of food. The further north one goes, the longer the day length and bigger the spike in plant growth.

All good things must come to an end and the movement of the earth around the sun ultimately leads us to autumn with ever-shortening day lengths. Plant production decreases as light becomes less available and temperatures fall. Insect abundance declines. Many migratory breeding birds depend on insects so must leave for southerly areas to avoid starvation.

The cost of migration is more than paid for by the abundance of food in the summer at higher northern latitudes. But not all tropical birds migrate. What are the costs and benefits of staying put? The benefit is the energy saved by not migrating. The cost is that competition for food is very high in the tropics and destruction of nests by predators is very high.

Tropical birds typically have multiple nests each season with a modest number of eggs, most of which are doomed to failure.

Hog Island Audubon Center

A landmark of environmental education since 1936, Hog Island Audubon Center's residential programs educate adults, children and families about coastal wildlife in Maine. The sessions are based on a 330-acre island in midcoast Maine's Muscongus Bay.

The sessions are led by some of the world's most respected naturalists and environmental educators. You can find more information about their offerings at: http://www.maineaudubon.org/explore/camp/hi_overview.shtml.

I want to call your attention to a new session called Family Birding Adventures geared for families with kids between the ages of six and 13. One of the highlights of the week will be a trip to Eastern Egg Rock to see nesting Atlantic Puffins. The session will be led by Jason and Laura Guerard, naturalists from the Cape May Bird Observatory.

Jason and Laura met on Hog Island and Jason later proposed to her there. For more information, visit the Web site above or call (888) 325-5261, ext. 215.

Maine Butterfly Survey

The Maine Butterfly Survey, a five-year project to map the distribution and abundance of the butterflies and skippers of Maine, has begun its second year. This project relies heavily on volunteers. The first year yielded more than 1,500 records (specimens or photographs).

Volunteers established four new state records and a large number of county records in the first year of the project. Two volunteers photographed Pipevine Swallowtails in late September 2007. These two records represent the first records of the species since 1907. You can see all of the 2007 records at the MBS Web site: http://mbs.umf.maine.edu/.

As the Volunteer Coordinator for the project, I would like to encourage you to participate in the project. All participants are asked to attend a training workshop. Each workshop participant will be given equipment, a handbook and voucher forms.

The last 2008 workshop will be held on Saturday, June 21 at Colby College in Waterville. The workshop will begin at 9:30 a.m. and conclude around 2:30 p.m. Lunch will be provided. If you are interested in attending the workshop, please e-mail me to reserve a spot. Enrollment is limited.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu. Previous columns and other information on Maine birding can be found at his blog: http://www.mainebirds.blogspot.com/.