Bird Behaviour: Migration
While migration occurs in other animal groups it is most pronounced in birds. Broadly speaking, bird migration is a regular movement between the breeding areas and those which are best suited to feeding needs at other times of the year. There has been much discussion as to the origin and significance of migration. However, it is generally agreed that it enables birds to exploit food that would otherwise be unavailable. Also young can be reared in areas that provide sufficient food in the appropriate season but which may be very inhospitable at other times of the year. Since most northern species breed during the summer months migration takes place in the spring and autumn. It is possible that this type of migration may have evolved after the last glaciation in response to the climatic changes which occurred.
Living as we do in the northern hemisphere we tend to think of it as ‘home’ for migrants returning from the south. However, having recently spent a winter in a warmer and more southerly part, just north of the equator, where many northern migrants were ‘wintering’, I tended to take a rather different viewpoint. Many of our summer migrants, which are primarily insect-eaters, spend an average of only four to five months with us. Swifts may remain for as little as three and a half to four months and Spotted Flycatchers and others for about four months. So that for the greater part of the year they are either travelling or in their winter quarters.
Most birds are migratory to some degree. There is every gradation between really sedentary species and those, like the Arctic Tern, which travels some 18,000 kilometres from the Arctic Circle to the Antarctic. Some birds may only make a purely local movement during the winter months like the adult Greenfinches which wander around within a few kilometres of where they normally nest, although young Greenfinches can travel several hundred kilometres.
The classic long-distance migrant is the Swallow. It breeds over much of the northern hemisphere and those from temperate Europe winter south of the Sahara in Africa, often retaining their geographical grouping. The British population concentrates on the moist south-east region of South Africa. The timing of the Swallow’s arrival in Britain probably depends upon the availability of certain types of flying insects. The Swallow has always attracted attention and for generations its return has heralded the spring, even in the days when it was thought to hibernate under water. It is a day-migrant and its movements are so obvious that we can trace them Northwards in spring and southwards in autumn from the records which are kept by many people all over Europe. Peter Davis showed, in 1965, that the main direction of the autumn passage of Swallows through Britain lies to the south-east and south – a direction which generally leads them to the shorter Channel sea-crossing and avoids the longer crossing towards Spain. This direction is maintained into Europe, but some apparently turn south or south-south-west after reaching north France. Like so many other species which winter south of the Sahara or Libyan desert most Swallows cross the thousand or so kilometres of the western Sahara.
On their return northwards Peter Davis’s analysis of the Swallow-ringing recoveries indicate that they tend to retrace the path of their autumn migration although the final stages of the spring return may pass further north and east to Belgium and the Netherlands and the final direction of the spring movement into Britain and within Britain is mainly to the north-west and northwards. Indeed, in west Cambridgeshire my own rather limited observations show Swallows crossing my village in a southerly direction in autumn, which leads them to the shortest sea crossing. In spring they travel in a northerly to north-westerly direction. Sometimes geographical factors can divert these birds from their preferred direction. A study of Swallows and other day migrants in your area could show how local geographical factors affect the direction of their flight.
Many different species of birds such as Redwings and Fieldfares which breed to the north and east of us winter in the British Isles unless the weather is so hard that ice and snow cover the land. Then the birds are forced to fly yet further south and west in search of food. Other species use Britain as a staging post, and many of our estuaries are used in this way by waders from the north. The Barnacle Goose which winters here is interesting in that it has three populations which have quite distinct breeding and wintering areas: the Greenland population winters on the north-west coasts of Scotland and Ireland; the Spitzbergen population winters in the Solway and the Russian population in the Netherlands. There is no evidence of any large-scale mixing, even in their winter quarters which may be as little as a hundred kilometres apart.
How do the birds which undertake these long migrations know where to go and when they should return? Although orientation and navigation in birds has been much studied recently, the answer is still unclear but it is known that birds find their way by a form of compass navigation.
In an experiment to determine how birds navigate a German scientist placed birds in an octagonal cage with a window in each of its sides. During the autumn migration period he counted the number of times birds hopped on to perches on each of the eight sides of the cage. In this way he discovered the preferred orientation of the birds if they had been free – for Starlings this was chiefly to the south-west. This is known as the primary or standard direction. To test the validity of the results he fixed mirrors to the windows in order to change the apparent direction of the sun. As a result the Starlings hopped on to perches on what was now the apparent southwestern side.
Dr Geoffrey Matthews went further and showed that birds were apparently able to navigate by using the sun’s position combined with an innate sense of time. Experimenting with Manx Shearwaters on the island of Skokholm, he was able to show that birds released in areas in which it was unlikely to have any prior knowledge were, when the sun was shining, able to orientate themselves rapidly in the direction of Skokholm. When the sun was obscured the birds headed out in random directions and took much longer to return. There has been much argument over the years about the role of the Earth’s magnetic forces in the orientation of birds, and there is still no consensus of opinion.
Much of Dr Matthews’ work was done with birds making use of sunlight. However, it is also thought that many of the night migrants can orientate by the stars. Problems arise when stars are obscured in the course of a migratory flight. Radar observations have shown that it is possible for birds to continue on their line even though the stars have been obscured by cloud. If, however, strong winds arise the birds may then become completely disorientated and be blown off course. This results in what the late Kenneth Williamson described as ‘drift migration’. In autumn, for instance, birds may begin their autumn flight in a south-westerly direction under anti-cyclonic conditions with a clear sky. Should they meet cyclonic conditions with strong winds and overcast skies they may lose all points of reference and drift down wind. It is this sort of drift which often results in the exciting migrants turning up on the coasts of Britain.
Seabirds, like landbirds, migrate in order to avoid the scarcity of food. Some birds such as gulls, Razorbills and Guillemots, spend most of the Winter on the continental shelf, while others like albatrosses and Puffins roam far out to sea. Some species seem to have a fairly definite wintering area but the movement of other species such as Little Auks, and petrels are little known and may be affected by the wind direction and strength.
To take a few examples, Herring Gulls, during the winter, wander locally around the coast of Britain, probably not travelling more than 2-300 kilometres from their nesting areas. On the east coast the numbers of local birds may be swollen by visitors from north-western Europe. On the other hand the Lesser Black-backed Gull, which is closely related to the Herring Gull, is chiefly a summer visitor to Britain and in winter travels to the western Mediterranean and the Atlantic as well as Europe and Africa as far south as Nigeria.
The most remarkable migration is that of the Arctic Tern. It nests along the north coasts of Europe, Asia and North America and spends our winter in the southern Atlantic and in the Pacific and occasionally even below the Antarctic Circle. The American breeding population crosses the Atlantic in autumn before turning south and then moves down the eastern half of the Atlantic, having joined up with the northern European population. Finally, some of these birds reach the Antarctic pack-ice and spread out along its edge.
The Manx Shearwater, which was originally studied by R. M. Lockley on his island home in Pembrokeshire, is also a great traveller in winter, and ringed birds have been discovered along the South American coast as well as in Australia, but the complete story of its winter movements is still to be discovered. Of course, many seabirds, and particularly the auks and their young, swim quite long distances in the autumn when they leave their breeding quarters.
What you, the naturalist, see of migration depends on many factors, especially the geography of the area in which you live. If you look at maps in some bird books which show the directions which migrants are said to take, you might think that they follow rather narrow routes, perhaps down the coasts of Britain and Europe, although we have seen that there is a tendency for Swallows to do this. But observations by radar have shown that, providing there are no impediments, migrants generally travel in primary or standard directions on a broad front across the country, although quite often rivers, hills, woods and coastlines deflect them in other directions, sometimes even in the opposite direction to the primary one. Many species such as the Wheatear, for instance, on their way from Africa to Alaska, must obviously change their primary direction on different parts of their routes. In western Europe the primary direction of most birds is south-westerly or southerly which takes them clear of the high mass of the Alps and towards the shorter sea crossing of the Iberian peninsula. In eastern Europe the primary direction of many species is to the south-east.
The Red-backed Shrike is famous for its ‘loop’ migration. R. E. Moreau, who made a special study of the trans-Saharan migration of birds, said that it is probable that in autumn the Red-backed Shrike of western Europe travels in an easterly or south-easterly direction until it nears the Aegean Sea when it changes to a southerly direction and crosses the eastern Mediterranean. In the return spring passage the shrikes leave Africa some 1000 kilometres east of where they entered in autumn, coming up north to the east of Suez and through Syria. There has been much discussion as to why they should undertake this migratory route. Moreau suggested that during the rather formidable desert crossing northwards the shrikes take advantage of the south-westerly winds that predominate at high levels and bring them into the eastern corner of the Mediterranean where the winds over the sea are also favourable.
In some parts of northern Europe where, for instance, the coastline of the Netherlands concentrates masses of Starlings, finches and Lapwings moving out of eastern Europe, one can see some huge numbers of birds passing. During one such movement on the shores of the Issjelmeer in the Netherlands, I saw 56,000 Chaffinches and 26,000 Starlings passing over in a matter of four hours. Only very rarely can you see such a volume of migration in Britain as the largest mass of birds from north-east Europe and Scandinavia generally passes to the south of us along its primary southwesterly direction. However, on the west coast of Wales and in one or two other areas, which concentrate birds, you can see some fairly striking movements. Throughout Britain on October and November mornings, shortly after sunrise, it is possible to see Chaffinches and other birds moving in a south-westerly direction.
The first autumn movement in Britain is, perhaps, less obvious than many others, it usually begins about mid-June, when Lapwings, dispersing from their breeding grounds, fly westwards in small flocks. Travelling at the same time may be Curlew and other marshland waders. This is not a migration in the strict sense but more a dispersal of young birds and unsuccessful breeders. They pass over my part of Cambridgeshire and most go further west since, at this time, the cereal has not been harvested and we have little pasture land.
What has emerged from a number of studies of birds’ behaviour and movements after the breeding season is that migrants may not stay in one place throughout the winter but perhaps stay for a period of a month or so in one area before passing on to another. Waders may spend several weeks in some estuaries fattening up before moving on to other appropriate habitats further south. It has recently been shown that Pied Flycatchers congregate in the Iberian peninsula for a ‘fattening’ period where they prepare for the trans-Saharan crossing. We all know that our own Redwings, Fieldfares and, indeed, visiting Blackbirds and Song Thrushes may stay with us in our local fields for a period before they too move on to other parts, sometimes further south if the weather with us is very hard. A great deal depends upon the availability of food as well as the weather and, as a result, birds will travel by different routes in different years.
Hard weather starts off very large movements to the south-west and south. For example, one January, after heavy snowfalls on the Pembrokeshire coast, in a matter of fifteen minutes, I counted 2,600 birds, mostly Starlings, Skylarks and Lapwings, flying westwards towards Ireland. Later in the same day I calculated that in one area of about five square kilometres there must have been over a million birds foraging on the fields. From time to time I have seen, even in Cambridgeshire, smaller movements of birds in westerly directions after substantial falls of snow either to the east or north of us.
In recent years, radar has increased our knowledge about migration and made it clear that we, with our binoculars, only see a fraction of a movement. What we may see on the ground has therefore to be interpreted most carefully. Nevertheless, the local birdwatcher should know the primary or standard direction followed by the common visible day migrants, and try to find out what happens to them as they move through his part of the country and subsequently compare the numbers that he has seen, as well as the directions, with any previous published records. There is still scope for some co-operative efforts amongst members of bird clubs in different parts of Britain to trace what is happening to our local migrants.
Finally, coming to some technical points: in recording the migrations of birds in your notebook, there are a number of things to note. Inevitably, you should record the date, place and time during which the observations took place, as well as the weather conditions, including cloud cover, wind direction and speed according to the Beaufort scale. You can record the flocks as they appear and pass over your observation line. You need to record the species, the number in each flock, direction of flight, height above ground and any calls that they are making. If you have time, try to record comments on any other features that strike you, whether, for instance, the birds continue to fly in the same direction.
Is it true, for instance, that Chaffinches, while travelling in their primary direction, tend to head from one wood to the next? It is particularly important if you are near the sea and are watching birds take off over the sea to follow them for as long as you can. Do they change direction after they have gone some distance from land? Do they gain height or go down closer to sea-level? On good days it can happen that the birds are passing in such numbers that you cannot record all the details in full. If you have a miniature tape-recorder you can record your observations and transcribe them later. Failing that you can draw a compass-rose with eight or sixteen points, in your notebook for each species that is passing. To each of the directions you add an arrowhead with the numbers for each flock – it is best to use a new compass-rose every hour or so, or more frequently if required. While this is a quick way of coping with large numbers you do lose some precision. If you visit different locations in your birdwatching area this sort of compass-rose with flight directions marked on, can summarize your observations of the general direction of migration in your area and, perhaps, can show how various geographical features effect the routes.
I used to map the autumn migration of Swallows and Meadow Pipits on Skokholm and later on also, the Skylarks on the Dale Peninsula. Building up an accurate picture of what is happening to birds within your area is a useful and satisfying occupation, and perhaps one day that knowledge may be required by others.