How migration is studied
Until about 100 years ago we knew very little about bird movements. A method to record bird movements was eventually devised. This involved putting small metal rings onto birds legs. Each ring carries a different number and each bird becomes a known individual. Birds are caught at special trapping places, ringed and released. They live normally, some for many years.
Most of the birds that are ringed are never seen again, but over the years enough birds have been recovered to enable us to learn a lot about migration.
When birds ringed in Ireland are found at various points along their migration routes their presence is recorded. Some birds are found dead and their ring numbers sent to the British Trust for Ornithology (BTO), which organises the UK's ringing scheme.
Many ringed birds are found alive, often at special trapping points where they were ringed perhaps a year earlier. This gives information on how long the birds have lived and we can tell how old a bird is if it was ringed as a fledgling. This is how we know that Swallows can travel to South Africa and back for up to 16 years. We know they go to South Africa because their ring numbers have been recorded by South African birdwatchers and because birds have been recorded in the UK carrying South African rings.
Birds are also found in this country bearing German, Scandinavian and Russian rings, among others, and birds ringed in Ireland are often recorded in these countries. This is how we know about bird movements around the world.
Bird rings are specially designed by the BTO and only specially trained ornithologists are licensed to trap the birds, handle them and place the rings on their legs. The rings are made of lightweight metal and come in different sizes to suit different birds.
Rings found on racing pigeons or caged birds are not part of this scheme. They are coloured, and are used to identify the owner of the bird. A bird in a race may also be wearing a rubber ring with its race number. BTO rings are put onto wild birds to identify individuals, not to claim ownership.
Finding a ring
Any dead bird can be checked carefully, without touching it, to see if it has been ringed. If you do find a ringed bird, send off the details either to the address on the ring (usually the British Museum), or to the British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, England. Include the ring number, copied very carefully, or better still the ring itself; the date; details of where you found it; and any evidence about how the bird might have died. All reports are useful, and the BTO has an excellent system of sending information about the bird to the finder.
Evidence for this came unexpectedly during the Second World War. Radar was in its early stages of development and operators were mystified from time to time by strange echoes showing on the screens. They did not correspond to aircraft and were nicknamed 'angels'. Some were caused by water droplets and other atmospheric phenomena but most were caused by flocks of birds on migration!
This eventually provided valuable information about the movements of migrants. It showed that, except during bad weather, most migration was of a 'broad front' nature and took place at quite a considerable height above the ground, well out of sight of observers. Radar also showed that migration 'streams' changed direction several times during a single night in response to changes in the weather.
Modern radar is now so refined that individual birds can be tracked and the pattern of their wing beats shown on the screen!
The data on routes and times of migration are studied along with information about weather conditions and the geographical features encountered by the birds. We now understand more about why birds take a particular route and why they choose to fly at a particular time.