Bird Migration 101
In everyday speech, migration is regarded as the mechanism behind the seasonal appearance and disappearance of some species of birds, mammals, fish and insects.
To most people however, migration equals bird migration (or perhaps mammals too).
Though in fact many insects, some mites and spiders, some reptiles, amphibians and even plants migrate regularly.
Here though we are only concerned with the migration of birds.
What is Bird Migration?
A more exact definition of migration is:
“the mass intentional and unidirectional movement of a population, during which time normal stimuli are ignored.”
This allows migration to be distinguished from dispersal. Dispersal is multi-directional, often only involves part of a population and does not involve the ignoring of normal stimuli.
This bit about ignoring normal stimuli means that (whereas under normal, non-migratory circumstances a species will respond to some stimuli, such as a flower, tree or another member of the same species in a particular way, e.g., by stopping its flight and feeding, or showing aggression) during migration these responses are suppressed and not allowed to interfere with the forward momentum of the migration.
Remember, normal stimuli can be suppressed at other times as well and some responses are seasonal in themselves.
Therefore the mere suppression of a response to normal stimuli does not indicate migratory behaviour on its own. However, combined with continual movement in one direction of all observed members of a population, it is a good indicator.
In birds, migration is seasonal and often spectacular and much is now known about the physiology as well as the mechanics of migratory flight.
Bird Migration History and Myths
Mankind has probably been aware of the fact of bird migration for many thousands of years.
Birds such as ducks, geese, swans and quail were valuable food resources to early man. Their tendency to be plentiful at one part of the year and scarce or non-existent in another would have been noticed and recorded in tribal law. Though the fact that the birds actually migrate, and why they do, remained a mystery for many years.
The bible and ancient Egyptian tomb paintings are perhaps the oldest recorded references to bird migration. The reference in Numbers 11:31 and 11:32 probably applies to a quail migration. Similar migrations still occur around the Mediterranean today. Other biblical references which appear to be reference to the seasonal nature of bird migration include Jeremiah 8:7 and Job 39:26.
The first natural historian to write about migration as an observable fact was Aristotle.
Though Herodotus did describe the migration of Cranes from north of the Black Sea to Central Africa (with some fancy embellishments) 100 years before.
Aristotle was an astute observer and as well as recording the times of departure of some species from Greece, and listing Pelicans, Turtle Doves, Swallows, Quail, Swans and Geese correctly as migrants, he accurately observed that all migrating birds fatten themselves up before migrating.
A fact that was subsequently ignored for 2,000 years.
As much a genius as he undoubtedly was, Aristotle made some serious blunders in his theorizing about birds. In Greece, Redstarts migrate south in the winter, however Robins from further north migrate south to Greece. As some redstarts would have been moulting before migration, losing their bright breeding plumage, it is easy to see how he became convinced that Redstarts change into Robins for winter and back into Redstarts for summer.
He called this “bird transmutation”. His second error was to report that swallows hibernate in holes in trees in a torpid and featherless state. We now believe that this theory arose as a result of observing late breeding swifts in Autumn.
Newly hatched swifts are not much smaller than an adult plucked swallow. Swifts will nest in holes in trees and young swifts are one of the few species of birds capable of entering and emerging from a torpid state. This is an adaptation to the fact that sometimes both adults have to be away from the nest for hours – or even days – in the search for food. This is particularly so during bad weather when flying insects are hard to find.
After Aristotle, little was done except to publish rewritings of Aristotle’s original works – even Pliny was basically just a copy of Aristotle. A few writers made original observations such as Frederick II of Germany in his book ‘On the Art of Hunting with Birds’ (though he obviously wrote the title and the book in Latin not English).
In 1251, Matthew Paris writing in Hertfordshire recorded what is the first reference in England of the migration of Crossbills. By the 1600s good evidence had been supplied by the French ornithologist Pierre Belan to refute many claims of hibernation, by the simple act of keeping the supposedly hibernating birds in a large aviary supplied with all the facilities it was claimed they needed to hibernate. None ever did.
However despite this – and a few other rational writings – many authors continued to support the hibernation theories. Even pretending to have observed swallows being drawn up in nets with fish from the bottom of lakes (where they hibernated).
Even up to the 1800s, supporters of bird migration as a concept held some ideas we find very strange today. Such as that birds migrated to the moon, or that they simply circumnavigated the planet in a series of endless circles.
In 1808 Forster published a work showing up the foolishness of the hibernation claims and from there the science of ornithology moved steadily forward. The migration of birds is now a well known and accepted fact and we find it hard to understand how such fanciful ideas could ever have been believed.
It is important though to remember when considering the errors of our ancestors, that until the 19th century optical equipment was extremely rare, bird identification guides non-existent, travel to other countries difficult (and expensive) and bird ringing, of course, had not been invented.
As a final controversial note, to show us that nature is wonderfully diverse, in 1946 the Nuttalls Poorwill Phalaenoptilus nuttallii was found to be a bird that actually does hibernate, it does so in the Colorado Desert, California where it lives.
More strange beliefs about birds – which were common for one reason or another – include that Barnacle Geese turn into barnacles for the summer and vice-versa. This belief was supported by the general populace because it allowed Barnacle geese to be classified as fish which could thus be eaten on Friday and during lent!
The recognition that returning migrants indicate the end of winter is encapsulated in the old saying:
“one swallow maketh not a summer”
Which is a saying echoed in many other languages. It was also believed in many cultures that small birds, such as warblers, migrated by hitching a ride on larger birds such as cranes. Cree Indians in what is now USA had the name napite-shu-utl to describe birds which rode on the backs of the Sandhill Crane.
Scientific investigation of bird migration began in 1802, when Audubon first began labeling birds with metal leg bands.
It was not until this century, when large numbers of bands with printed numbers and letters became available, that this method really began to deliver results. The first mass produced bands were made of nickel, which proved unsuitable because of its tendency to oxidise and become unreadable.
Nowadays banding rings are made of an aluminium alloy called manel or incaloy. The numbering of the rings is controlled by a national body in most countries and the rings have a contact address on them.
These national bodies co-operate with each other in exchanging information on banding records (either live caught or found dead) of birds ringed outside the country in which they are caught.
Banders or Ringers – as they are called in the UK – have to work hard to get any returns. Especially with small birds, the number of banded birds which are either re-caught or found dead is very small.
Hundreds of thousands of birds are banded around the world each year, both by professionals and amateurs. This dedicated work by relatively few individuals has over the last 20 years or so generated a lot of useful information.
This information, in conjunction with that from radar observations and the collecting of exhausted and dead migrating birds from buildings such as lighthouses into which they tend to crash, has revealed most of what we know today.
When do Birds Migrate
Birds tend to commence migration in large numbers only when they have a favorable tail wind.
In North America the winds north in spring and south in autumn are ideal to assist seasonal migrations. Once started however, only very bad weather will stop them.
Many birds fly high when migrating because of prevailing winds at higher altitudes – and also because the cold at these altitudes helps them disperse all the heat being generated by their flight muscles.
Many species of wildfowl fly at 6,000m and some have been observed flying at 8,000m, 86mph in temp = -48 degrees C.
Not all migrating birds from a summer breeding site overwinter at the same area.
What happens, come autumn, if a male bird meets a female bird in the breeding grounds who has a different overwintering site?
Whose site do they go to now they are a pair?
In many species the pair bond breaks up at the end of the breeding season, but some like swans mate for life. In the case of the Bewick’s Swan, the male decides where to fly to for the winter and the female follows him. However, the female decides when it is time to travel back to the tundra for another year’s breeding.
The reverse scenario is when birds with different breeding sites overwinter in the same area. If pairing commences on the overwintering ground, whose breeding ground do they return to?
The answer may be different for different species. The only example I know of involved Mallards in the USA and in this case the male followed the female.
Timing of migration is a mix of internal stimulus – which results in a feeding binge to put on fat to survive the journey – and then the tendency to aggregate into flocks. Once the pre-migration flock is gathered, the feeding continues while the birds wait for suitable weather conditions.
Thus while the birds’ internal clock probably releases the hormonal triggers at a fairly accurate date each year, the availability of food and the presiding weather conditions decide when the migration starts and hence when we see the first spring migrants arrive and the last autumn ones leave.
A 12-year study of Common Terns at Cape Cod showed that an average 75% of birds, and as much as 83%, returned to the same area to nest in successive years. Eighty percent nested within 25 feet of the original nest site.
Another study of Layson Albatrosses showed that in the following year a nest was on average only 13 inches away from the previous nest.
Migratory Routes or Flyways
For geographical reasons, i.e. mountains, coasts and rivers, many migrating birds travel certain general flyways or routes.
Interestingly, in 1979 flyways bought 129 nations together.
In the USA there are 4 main flyways and these are termed the Atlantic, Mississippi, Central and Pacific flyways. In Europe, the mountains do not run so regularly north/south as in the USA. Therefore certain passes become funnels through which many migratory birds pass. Also the Mediterranean is a major obstacle to many birds and they veer either left or right to avoid crossing it.
Migratory routes are not fixed eternally and in some species part of the population follows one route and part another. Also, some birds travel south by a different route to that which they use to travel north, e.g. Golden Plover.
Some migratory birds fly very long distances.
Some arctic terns fly 11,000 miles each way. Other birds fly lesser distances. Blackpolls from Hudson Bay overwinter in Venezuela, 5,000 miles each way and Golden Plovers fly 2,400 miles each way in the USA.
How Fast Do Migrating Birds Fly?
Birds often fly faster when on a migratory flight than they do during ordinary flight.
Thus distances of 200 to 400 miles a day are commonplace among long distance migrants. Some birds, however, migrate more slowly, e.g. Robins coming up the Gulf coast average 13 miles a day.
Most flights occur at between 600 and 5,000 ft above sea level, with an average height of 1525 ft a.s.l.
However, mountains may mean greater heights are needed – and heights over 10,000 ft a.s.l. are not uncommon.
How Do Birds Migrate
Little is known about how birds navigate.
Experiments show that most migratory birds have a built-in sense of direction and know innately which direction they need to travel. First year Starlings in Europe – kept in a covered cage and away from birds which have already migrated once or more – still move to the correct side of the cage when the time comes for them to migrate.
Some birds appear to use landmarks and obviously at a height of several thousand feet they can see a considerable distance.
Young crows born and raised in Alberta – and then kept caged until after all the population had flown south and the first snows had fallen – flew straight to Oklahoma where the rest of their flock was.
Young birds can also learn a lot from travelling with their parents. Mallards are migratory in Finland, but not in England. Young hatched from eggs taken from English Mallards and put under Finish females had no problems migrating with the rest of the population.
Some migratory birds, particularly those flying across the North Sea, navigate at least partially using wind direction.
This makes sense when you realize, that in many places, the wind blows in the same direction in the same season every year.
A number of elegant experiments involving displacing birds to different geographical regions, have shown that many birds use the sun (at least during the day) as a cue to direction when migrating or homing.
More clever experiments – some involving rearing birds in cages so that they have never seen the night sky and/or putting them in a planetarium – have shown that many birds orient themselves by means of the stars, but not the moon or planets, and that this ability is innate.
Birds are known to stop migrating on cloudy nights.
Birds of prey, swallows and crows migrate by day. Thrushes, warblers, cuckoos and woodpeckers migrate by night. Wildfowl migrate both day and night. Most songbirds migrate at night.
There is believed to be some hormonal stimulus to migrate, resulting (at least in the spring), in the development of the gonads.
Other stimuli appear to involve temperature, daylight/darkness ratios and an internal clock.
Well, I hope you have learned a little more about the incredible world of bird migration. Perhaps now you should check out our page on bird nests.
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