Discovering: Migration part 1

This is the first in a series of articles on the fascinating phenomenon of migration.

Migration is the periodic movement of animals to and from different areas. Usually the distance travelled is relatively long when considering the size of the animal. There are a number of reasons why a species may migrate, the most prevalent reason is to find suitable places to breed. This suitability is often linked to seasonal differences in food availability.

Bird migration

One of the most obvious and well loved of all migrations is bird migration. Even in Biblical times migration was noticed and appreciated “” Even the stork in the sky knows her seasons; and the turtledove and the swift and the thrush observe the time of their migration; but My people do not know the ordinance of the LORD.” Jeremiah 8:7 (NASB).

In the Northern hemisphere, many species of bird migrate north in the Spring. These birds spend the Winter in warmer climates of the tropics or equatorial regions where food is abundant. Come Spring, triggers such as the lengthening of days, increased reproductive hormones, and inherent restlessness termed zugunrhue cause the birds to migrate as an innate response. Before migrating the birds need to prepare for this long and perilous journey. Muscles enlarge, while other organs reduce in size. Fat, which is needed for energy to fuel the flight, is deposited in the body, with some small birds doubling their body weight prior to migration. This energy reserve is especially important if the birds are to cross major deserts, where stop-off food supplies are far from certain.

Navigation and weather

To navigate, birds make use of their high vantage point up in the sky to use visual cues to guide them such as landmarks (coastlines, mountain ranges, waterbodies) and birds may navigate at night by using the stars. Young birds which lack experience often travel with adults on their first migration, but sometimes, when adults leave first, young birds of some species still manage to migrate to the correct breeding grounds, as they were born with the information needed to make the journey. Thus showing the care of the Creator for His creation. As some birds migrate using the stars, some prefer to migrate on cloudless nights, meaning weather is an important factor in migration, which is also evidenced in passerines (songbirds) using a south wind to help carry them across the vast Sahara desert.

Risk

Migration must be worth the risk. The energy expenditure for migration is huge, and many birds have to migrate through areas known as geographical bottlenecks where they are especially vulnerable to predators. For example, the Eleonora’s falcon (Falco eleonorae) times its breeding to coincide with the migration of small birds across the Mediterranean which is a geographical bottleneck for passerines (songbirds). Also, across the Mediterranean, legal and illegal hunting results in the deaths of thousands of migrating birds each year, especially in Malta and Egypt. 

Differential migration

In some species, males and females migrate different distances. Female dark-eyed junco (Junco hyemalis) migrate further south in the Americas than males. The females winter in areas with a warmer climate and as a result, have a higher over-winter survival rate than males. The reason why the males winter further north is because they are taking a risk that can benefit them come the Spring time. If the males that take the biggest risk in wintering closest to their breeding grounds survive, they have a shorter distance to migrate in the Spring, thus arrive at their breeding grounds earlier. A strong positive correlation exists between early arrival at breeding grounds and increased productivity in numbers of offspring.

Extreme migrations

Some birds make extreme migrations. Some travel extremely long distances, some make extremely long non-stop flights, and some travel over and across extreme terrain.

The longest migration is carried out by the sooty shearwater (Ardenna grisea), which makes a round trip of 65,000 km each year. This small sea bird, migrates across major oceans of the Pacific and Atlantic. The breed in South Pacific islands such as the Falklands, New Zealand and Tierra del Fuego, and in Winter they can be seen off the coast Britain.  

Another long distance migrant is the Arctic tern (Sterna paradisaea). In this classic example of migration the Arctic tern migrates from the South polar seas to the North polar seas and back again each year, a distance of around 80,000 km.  Also worth noting is the migration of the ruby-throated hummingbird (Archilochus colubris). This tiny bird doubles its weight from 3 g to 6 g to make the 800 km 20 hour non-stop crossing of the Gulf of Mexico. 

The longest non-stop flight is carried out by the bar-tailed godwit (Limosa lapponica). This large wading bird migrates from the Arctic to Winter feeding grounds in New Zealand, Australia and Southern Africa. Before migrating, the birds move from the tundra where they breed to estuaries which are rich in food such as crustaceans and worms. In 2007 one female bar-tailed godwit flew non-stop from Alaska to New Zealand, a distance of 11,570 km in just over 8 days. Her movements were tracked by a transmitter that scientists placed on her earlier in the year.

Some birds make equally as impressive journeys, but not always intentionally. Every Autumn, some birds migrating from North to South America have to face storms, some of which have very strong winds, which can blow birds off-course. Some of these small birds such as diminutive black-and-white warblers (Mniotilta varia) find themselves in Europe, not south America as intended. This phenomenon of migrating to unintentional areas is known as vagrancy and also occurs when birds over-shoot their migration.

The final extreme form of migration is over extreme terrain.  Bar-headed geese (Anser indicus) fly over the peaks of the Himalayas on their migration between Central Asia and the Indian sub-continent. In the Winter, the birds migrate to India to escape the bitterly cold Winters of the high mountain plateaus, and in Spring they migrate back to the plateaus to escape the Summer monsoon rains of India. To undertake their extreme high-altitude fight, the geese have a number of  God-given adaptations. Firstly, their wings have a large surface-area to body weight ratio which provides the extra lift needed to fly high. Secondly, the birds haemoglobin (a protein in blood which carries oxygen), absorbs oxygen faster than other birds, and finally, the birds have downy feathers, which insulate against the very cold conditions experienced at altitudes of up to 10,000 m (the record flight height stands at 10,175 m).

The next part in this series will explore the mass migrations of land animals. 

Sources

• Animal Life, Charlotte Uhlenbroek, Dorling Kindersley, 2009. 
• Essential Ornithology, Graham Scott, Oxford University Press, 2010.