Birdwatching: Geographical Distribution of Birds
Amateurs have probably contributed as much to the knowledge of the geographical distribution of birds throughout the world as to any branch of ornithology. While the British Empire was still spread around the globe, the colonial servants, soldiers and sailors, as well as a number of travellers and explorers who probed the various corners of the Earth, were curious observers and keen recorders. The problem of identifying species and their geographical forms was very different to what it is today. There were few comprehensive handbooks at the turn of the century – the early ornithologists in their travels round the world, had to use the gun. They collected large numbers of specimens which they labelled and brought back home to form the present museum collections. With the help of these museums they gradually built up the picture of what particular populations and groups of birds formed species, and how they were distributed throughout the world. Some people may be rather horrified to think that birds might have been shot for scientific reasons but we must remember that taxonomy owes nearly everything to the museums and private collections of birds. Also without these collections we might not have the coloured plates of birds that are found in the handbooks and field guides today. There is, thankfully, very little need for collecting today in Britain and Europe and because of the original work and the collections of others we have books of such excellence that as amateurs we can pursue some of the aspects of the study of geographical distribution without having to resort to killing.
The study of the geographical distribution of birds includes firstly, the study of the complete world range of a species and its populations as they are today, as distinct from the study of its local distribution which may only depend on the availability of suitable habitats. Secondly, it includes the study of the way in which species have originated and how their distributions have changed.
One of the remarkable facts about the distribution of birds is that although they are able to fly long distances, their range is generally limited to specific areas of the world. A few species are fairly cosmopolitan: the Osprey, for instance, which now breeds in a number of places in Scotland and news of which hits the headlines almost every year, breeds in Europe, Asia, Australia, and North America and winters in South America and Africa. The House Sparrow is another which, chiefly because of man’s helping hand, has a world-wide range. The Barn Owl, now comparatively rare in this country, ranges widely over other parts of the world. At the other extreme, some species may be limited to one small island: for instance, a warbler on Aldabra in the middle of the Indian Ocean is entirely specific to that island. The Galapagos Archipelago, which comprises a number of small isolated volcanic islands in the Pacific Ocean, has several species which are restricted to one or more of them.
The distribution of a species includes both its breeding and non-breeding ranges which may be markedly different: Swallows and warblers breed in the northern hemisphere but winter in South Africa; Arctic Terns breed in the far north and winter well south of the Equator. In other species there may be a slight overlap in ranges: the breeding and non-breeding ranges of Robins overlap. Other species have a vertical element in their range – Snow Buntings nest on high ground in high latitudes but winter along coastal regions. Dippers tend to move downstream from the higher levels of the hills in winter.
If you examine details of the distribution of different groups of birds, various factors will begin to emerge; in particular you will notice that certain regions are characterized by the presence of species peculiar to them. The earlier ornithologists found that attempts to describe the avifaunas of the different regions in terms of traditional continents were not particularly satisfactory. For instance, they found that the Sahara Desert rather than the Mediterranean Sea constitutes a latitudinal barrier to the dispersal of birds in Europe. Similarly Europe and Northern Asia seem to form a single faunal region.
The Palaearctic region in which we live is one of the largest zoogeographical regions yet it is relatively poor in bird species with only sixty-nine families. It has only one unique family – the Prunellidae, the family to which the Dunnock belongs – but it shares forty-eight families with the Nearctic of North America. The richest area for bird life is the Neotropical region of South America with eighty-six families and over 1,500 species.
Quite recently the idea of ‘continental drift’ has been generally accepted by the scientific world and it is now thought that the continents as we see them today were once part of a ‘super-continent’ which have since drifted apart. This theory would explain certain peculiarities in the distribution of plants and animals. For instance, you might find that a certain group of animals have more affinity with a group on another continent than with the others on their own continent.
During the course of the evolution of birds, conditions on Earth have changed considerably, often from one extreme to another. Droughts, ice-ages, earthquakes and floods have all had their effect upon the face of the globe. Therefore birds must be able to adapt to their environment if they are to survive and as the environment changes they must be able to evolve to take account of the change, or they must be able to move elsewhere. The geographical distribution of birds is therefore the result of the gradually changing landscape and an evolving bird population. Birds have an inherited tendency to disperse from their place of birth and invade new areas. They are usually very mobile, though their power of flight can vary considerably. Crossbills and Nutcrackers manifest a form of dispersal following seasons of abundant food and a marked growth in population as a result of which large numbers erupt across the barriers of their normal range. Population pressure promotes range expansion in other species and it is often the young which travel furthest. Physical boundaries such as seas and mountains act as a buffer to these spreads. These barriers can sometimes be overcome; for example, various landbirds crossed the Pacific Ocean in large enough numbers to establish breeding populations in the Galapagos Archipelago.
Probably the decisive factors in determining the range of a bird are the climate and the nature of the soil – these two factors determine the vegetation and consequently the food, cover, nest sites and other important elements. The change in climate together with the changes in the habitat have probably been the cause of the decline in Britain and elsewhere of the range of the Red-backed Shrike and the Wryneck. The Cetti’s Warbler, which had invaded England from the continent will, however, probably be confined by the climate from spreading from the south of England. Another factor which can prevent the spread of a species is the presence of another species which has similar ecological requirements and will therefore compete with the new arrival.
The study of the ranges of birds is still a study in which a great army of amateurs participate, or can participate providing they submit accurate records to their county bird recorders. Many birdwatchers are interested in the unusual bird which turns up outside its range. Sometimes the appearance is an accident in that a migratory American bird might have been wind-drifted across the Atlantic or a bird migrating northwards to Scandinavia might have been drifted under overcast skies across the North Sea to the shores of Britain. On the other hand, the appearance of some rare species may be the forerunner of an invasion. The Collared Dove, for example, has become ‘dynamic’ in the last fifty to a hundred years and has swept across Europe from Turkey. It was first recorded in Britain in the early 1950s and has now, in a matter of twenty or thirty years, virtually colonized the whole of Britain. It has increased to such an extent that it is now considered by some to be a pest, and it probably holds some sort of a record for a quick change from inclusion in lists of birds especially protected into lists of pest species.
There are various other species whose range is gradually changing. The Serin, which is closely allied to the domesticated Canary, is another species which swept across Europe and is now on the other side of the Channel, and, although the occasional individual has been recorded on this side, they have not yet really established a bridgehead in Britain. Several species have re-established themselves in Britain in recent years, sometimes because their habitat has re-appeared. The Avocet, Black-tailed Godwit and Black Tern used to nest in the Fens of England over a hundred years ago until drainage destroyed their habitats. Changes in land use in recent years have once more produced the sort of terrain – rather wet – on which these birds can complete their nesting cycles, and the RSPB has acted to purchase and manage this land to ensure that this time they remain with us. Oddly enough, fewer species are ‘retreating’ from Britain at present than are colonizing.
The decline in the distribution of the Stone-curlew and the Wheatear in Britain is probably related to the activities of man. Their habitat is ideally of a steppe or semi-desert nature, which, in Britain, is only found naturally on the summit plateaux of high mountains. The presence of these two species in lowland Britain was mainly the result of the felling of forests and the introduction of sheep and rabbits to the land. The spread of myxomatosis and the decline in sheep numbers has meant that the heavily grazed habitat suitable for the two species has decreased. What propelled these species into Britain is not known, but I think that suitable lowland habitats for the Wheatears probably only appeared in the twelfth century.
The lists of birds that some of us keep when we go out in the field no matter where we are, can either be a waste of paper or can provide a record of value. If you put nothing but a tick against a bird’s name you are at least making some record. On the other hand, the more accurately you record your observations the more valuable they will be. The figures you record can become a base line against which you can compare figures you obtain from later counts in the same habitat. Records like these are often the basis of county bird reports which in turn have been used for more extensive reports, for example, John Parslow’s Breeding Birds of Britain and Ireland and the British Ornithologists’ Union’s The Status of Birds in Britain and Ireland. Therefore it is worth trying to add as much information as possible to your field lists.
Ocean transects are a valuable way of learning about the distribution of birds at sea, but sadly most people now travel by air. However, for those who do travel by sea the Royal Naval Birdwatching Society has special report forms which help to add a basis of uniformity to the records. Indeed, anyone who is lucky enough to be going on a long ocean voyage should get in touch with the RNBWS first and find out how he or she can contribute to this area of study.