I’ll Fly Home—Or Not

The snowy owl is sedentary, meaning it does not

migrate. The males are almost perfectly white,

while the females and juveniles can have black

barring. They sit, look, and listen for their prey,

which includes small rodents and even other birds.

Their hearing is good enough to let them target a

mouse under the snow from hundreds of yards away.

The Arctic tern travels from north of the Arctic Circle to Antarctica and back again every year. On the other hand, the Snowy Owl lives in the arctic region year-round; it doesn't migrate at all.

The Question of the Day:

Why do some birds migrate while other birds stay in one place?

The possible explanations are many. Maybe the type of food they eat is present only part of the year, or maybe they can’t stand the cold temperature. They might need to have their babies in a place away from predators, or perhaps migration is an evolutionary holdover that had a reason in the past, but no longer isnecessary.

How could you start to determine the reason for migration in only some bird species? I would start by looking at how closely related the migratory and non-migratory species are. Maybe all the birds that migrate are more closely related to one another than to the birds that don’t move around during the year. This would suggest that there is a genetic basis for why only some species migrate.

One research group did just this in 2007. The looked at 379 species of flycatchers, a closely related group of birds. They found that almost equal numbers of species were migratory or resident, so it doesn’t appear that genetic relatedness is the answer.

The painted bunting is among the most colorful

birds in North America. This male has several colors,

while the female is a brilliant green. They breed in

south central US or on the southern east coast, but

winter in Central America; therefore, they are

seasonally migratory.

Maybethe need to migrate has to do with the geographic region. About 90% of birds in the arctic migrate, some are present there only in the mid-summer months. The arctic tern is a good example. Arctic terns move with the summer, breeding in the arctic in May-July, moving down along continental coasts to arrive in Antarctica for the months of December to February. The entire distance traveled could be as much as 32,000 km (20,000 miles) in a single year.

Similar to the arctic region, the east coast of North America has species that migrate and species that are sedentary. About 80% of the birds species of the east coast move south during the colder months, but on the Pacific coast, almost all the bird species are non-migratory.

So migration is not due to the type of geography around the birds. However, the east coast of North America does have larger temperature swings than the west coast, so maybe it is just that some birds can’t deal with the cold. 

Some non-migratory birds can control the amount

of blood that travels to the legs in order to conserve

body heat. This works even better if they can reduce

the amount of contact with the cold surfaces, so some

of these birds perch on one leg at a time.

Manybirds that do not migrate have special adaptations to deal with the cold. Trying to keep a constant body temperature (endothermy) takes a lot of energy, and birds live right on the edge of having enough energy anyway. Flying requires a huge amount of energy and they must eat almost constantly just to keep enough carbohydrates in their system to be able to move around to find more food.

Burning more energy to keep warm might tip them over the edge into starvation. To alleviate this problem, many birds can allow parts of their bodies cool down to freezing or near freezing, while keeping their internal organs at a temperature that will preserve their function. Blood flow is a major way to keep parts of the body warm, a duck standing on the ice can reduce the blood flow to its feet and reduce the amount of heat lost to the cold ice. The duck’s chest may be 40˚C, but its feet could be just one degree above freezing.

But let us look again at the arctic tern. It migrates from the north polar region to the Antarctic region in such a way that it sees two summers each year. But these are summers in name only. The arctic summer has an average temperature from -10˚C to 10˚C, so much of the time the tern is there, the temperatures are near zero.

The arctic tern has a ghastly commute each year.

The trip is even more amazing when you consider

that during its yearly molting, the tern flies very

little.  So, all that distance must be fit in to just a part

of the year, not the entire 365 days. I guess they

vacation by NOT traveling.

Thenwhen they reach the Antarctic, the summer there has an average temperature of -2˚C to 2˚C. This is hardly a balmy vacation destination for the tern. The temperatures in both its breeding grounds and wintering grounds would require it to have elaborate temperature control and energy-saving adaptations. Therefore, inability to tolerate cold temperatures is not the reason for migration, at least not for many birds.

The group who carried out the 2007 study concluded that the main reason that only some flycatcher species migrate is not due to what they eat, or when they breed, or what is trying to et them, but to how available their food source is. Whether they are fruit eaters, or insect eaters, or seed eaters, how easy it is to find their food is the most common reason that migration has evolved for a specific species in a specific location.

The food availability hypothesis is supported by certain types of migration that are common in North America. Irruptive migration is characterized by a population moving to another place, but there is no yearly, seasonal, or geographic pattern. The birds may migrate one year, then not again for a dozen years, or they might go for several years in a row. North American seed-eating birds are famous for these migrations. The distance and number of individuals that migrate are also not very predictable, and this all makes it sound like the movement is linked to food availability. However, it could also be to escape some population explosion in a predator species or for some other reason.

It isn’t only birds that might undergo partial migration.

Some crab species will migrate for breeding purposes,

like these Christmas Island Red Crabs.  Individuals

that won't breed just don’t make the trip. They may be

too young, too old, too lazy. In other cases, when

populations migrate away from the breeding grounds,

some individuals may remain there the year round.

Anothertype of migration is partial migration, a pattern wherein not all birds of a species in a certain location will migrate, only some of them leave in non-breeding times, while others stick around year-round. Food may be available for some, but not all, or the environment may be unsuitable for some weaker individuals to have enough time to forage for a sufficient amount of food. These (and other reasons) might explain why partial migration exists, but one question remains, who stays and who goes? The choices could be based on age, altruism, suitability, dominance… laziness?

As an aside, it isn’t just birds that migrate. Mammals move from place to place, sometimes with a defined pattern during the year, but sometimes they just follow the food, a process called random migration. And some insects migrate as well.

For many years, the migration of the Monarch butterfly was believed to be the longest insect migration. But this is not the typical migration we think of, where an individual moves from one place to another and then back again. The migration of the monarch butterfly takes four generations to complete. Some generations are born and fly a long distance to lay their eggs, while others are born, live, and reproduce in a small area. But altogether, this butterfly moves from as far north as Canada to the high mountains of Mexico and back each year, about 7000 km (4400 miles).

Globe skimmer dragonflies breed in freshwater pools,

so they migrate from India’s monsoon season to the rainy

season in East Africa, all in search of a place to lay

eggs. They make stopovers on the Indian Ocean islands,

but only to rest, because there are very few

pools of freshwater on these coral cay islands.

A few years ago, a biologist in the Maldive Islands started to wonder about the movement of globe skimmer dragonflies where he lived. They seemed to be plentiful in some periods and absent in others. He started to track them, and found that they have an even larger migration pattern than the monarch butterfly. What is more, they fly long distances over the ocean with no place to stop and rest.

Over a series of generations, the dragonflies move from India to the Maldives, some 600-800 km across the open sea. Then they move to east Africa, from Uganda to Kenya and Mozambique. In January, they start back toward India, and complete their migration of more than 18,000 km (>11,000 miles).

So birds may get most of the publicity, but insects hold their own in the migration game. Of course, it does take four generations of dragonflies or butterflies to make their complete journey, where a single arctic tern may make its entire 20,000 mile trip thirty times in its lifetime. O.K., they are both pretty impressive when you consider most people need a car to go down the street to the grocery store and back.