A reader, Tony Scarpelli, wrote recently with some questions about the long migrations many birds undertake. We had an interesting exchange of emails on the topic, and I realized that discussion could make the basis of a column.

Attempts at explaining the reasons for bird migration are challenging. For behaviors that occur so broadly and over thousands of years, we cannot do controlled experiments to nail down the basis of migration. But we can make observations that are helpful in explaining why so many birds migrate.

First, we note that birds and mammals are the only animals that maintain a constant body temperature. So, the resting metabolic rate of a bird or mammal is much higher than a lizard, salamander or spider whose resting metabolic rate is set by the ambient temperature. Maintaining a high metabolic rate requires large amounts of food.

To understand the physiology of birds, we must recognize that food is everything. If common redpolls can find sufficient birch seeds on the edge of the arctic tundra, they can overwinter with temperatures reaching 70 below zero. With enough food, some birds can raise multiple clutches of six or more nestlings in a single breeding season. With sufficient food, arctic terns can migrate 36,000 miles in one year.

Because the earth is tilted on its axis, we have seasons with the northern and southern hemisphere experiencing summer six months apart.

Many of Maine breeding birds are migratory. They come from wintering grounds south of us to take advantage of the long summer days we have. During the summer, equatorial regions have 12 hours of sunlight while we have 17 hours or more. Our longer days allow plants to photosynthesize longer. The plants in turn support insects and other animals that provide bird food. It is easier to find sufficient food in Maine than in the tropics in the summer.

Of course, Maine in the winter is no place to be for these migratory breeders, especially aerial insectivores, and leaf-gleaning insectivores. So, moving south in the fall is mandatory. But how far south does a species migrate?

It depends on the availability of the food required. Yellow-rumped warblers may only migrate to coastal areas of the mid-Atlantic states while many other species migrate to Caribbean islands, Central America and South America.

Pick any winter day here and compare the day-length to areas farther south. Day-length will increase to 11 hours or more in parts of Central America, reach 12 hours at the equator and continue to increase in the southern hemisphere during their summer. Species like the bobolink that winter in Argentina essentially experience spring and summer twice a year.

Let’s do a little economic analysis of bird migration. For migration to be worth it, benefits have to exceed costs. Migratory birds incur the huge cost of fueling a migration but the benefit is sufficient food to get through winter. For temperate resident birds, the costs of migration are too high if food is hard to find.

Of the 700 species of birds that regularly occur in North America, 490 of them are migratory. That clearly shows the costs versus benefits of migration are worth it for most of our breeding birds.

As our climate continues to change rapidly, it is reasonable to think that bird migrations may not be sustainable. We know that migration behaviors can change rapidly. The blackcap, an old world warbler, provides a nice example. This European warbler formerly migrated south to winter in Spain and Morocco. In the 1960s, some blackcaps started to migrate to Britain instead. Now, about 10% of blackcaps choose the new wintering grounds.

All the current migration routes of Maine birds have been used for less than 10,000 years. The last ice age in Maine started to recede then when Maine was covered with 1.5 miles of ice!

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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