What is Migration?

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 migration is birds, and perhaps mammals. Though in fact many insects, some mites and spiders, some reptiles, amphibians and even plants migrate regularly. Here we are concerned with the migration of birds.

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 multidirectional, 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, say a flower, tree or another member of the same species in a particular way. For Example 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.

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 included Jeremiah 8:7 and Job 39:26.

The first natural historian to write about migration as an observable fact was Aristotle. Though Herodotus described 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 2000 years.

As much a genius as he undoubtedly was, Aristotle made some serious blunders in his theorising 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/do 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. 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 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 in 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 a saying echoes in 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.

Modern Understandings

Scientific investigation of bird migration began in 1802 when Audubon first began labelling 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 recaught 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 birds from buildings such as lighthouses into which they tend to crash, has revealed most of what we know today.

 



 










 

 

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