Although radar has become an extremely useful tool in the study of migration, the marking of a bird with a leg-ring, so that it can be individually recognized, has for a long time been a key method for discovering more about a bird’s travels, its life expectancy, its family relationships and so on. The ring is a very light, but strong, band of aluminium or nickel alloy. This band is carefully wrapped around the bird’s leg using a special pair of pliers. The ringer makes a note of the ring number, the species of bird and any other facts that he can glean about it, and then sends the most important details to the British Trust for Ornithology which co-ordinates the ringing scheme in Britain. A legend on the ring asks the finder to inform the BTO at Tring directly or the British Museum (Natural History), London, and in their turn they pass the record on to the BTO which has the data as to where the bird was caught, the number of times it has been caught, and how long it has lived. Details of each recovery are sent back to the finder and to the original ringer so that he too can build up a life history.
Birds can be caught in a variety of different ways. Some bird observatories, usually located on islands and peninsulars around the coast of Britain, still operate the ‘Heligoland’ trap. This is a huge funnel-shape construction covered in wire netting, which, as its name suggests, was originally developed on the island of Heligoland. The entrance to the funnel is open and is usually planted with shrubs which attract the migrants. The birds are driven from these bushes up the narrowing funnel and finally into a small catching box from which they can be easily removed. A second method of catching birds, which has been used for generations, is netting. The old nets were made of cotton and had a number of drawbacks. The use of nylon has made it possible to construct nets with threads so fine that they are difficult to see when set in front of trees and bushes. Because of this quality they have acquired the name ‘mist’ nets. They are extremely effective and in the wrong hands would do so much damage to birds that it is now necessary to obtain a licence to use them. Many ringers, particularly those working on a population of a particular area, ring nestlings. Nestlings, again, need considerable care when handling and a ringer has to obtain an additional licence. The advantage of ringing nestlings is that whenever one is recaptured, its exact age will be known.
The ringing of more elusive birds such as geese and waders has been carried out by the use of rocket nets which were developed successfully in Britain in the early 1960s by The Wildfowl Trust, Slimbridge. The method involves careful positioning of nets powered by rockets in high-tide roosting areas as well as attracting birds to the correct position for netting either by the use of decoys or careful shepherding from a distance.
In addition to the numbered rings there are other ways in which birds may be marked to make them recognizable as individuals. Some scientists mark the birds they are studying with wing-tags which are usually brightly coloured plastic tags. They are fitted to the forewing in such a way that they lie flat over the wing and do not impede the bird’s progress. Numbers written on them, together with the colours of the tag, enable the bird to be individually identified from some distance. They are safe if properly used but are unsightly – I can always remember seeing Jackdaws flying in to land from Skomer with what looked like orange headlamps attached to their wings.
When I studied the life history of the Wheatear, I wanted to learn as much as possible about the population. I therefore ringed as many Wheatears as possible not only with the BTO ring but also with three coloured plastic rings – two on one leg and one with the aluminium ring on the other leg. Each bird had a unique combination of colours so that I was able to recognize the individual, using telescope or binoculars, by checking on the combination. I learned a little about the migration of Wheatears – not many are recovered in their winter quarters – but I discovered that individual Wheatears will return year after year to the same territory, even to the same nest hole if the old nest has been cleared away, and even to the same mate if it has returned as well. I also learned something about their occasional bigamy; how the young dispersed after they left the nest; how many of the young returned to the island the following year to breed and with whom they mated. I learned how long some of them lived – one of my Wheatears was five years old and I would probably have found others of a greater age had I been able to remain longer on the island.
In the early days when there were few ringers, most of the birds were recovered dead but as more ringers took part in the scheme and worked on populations of birds in their area so the numbers of live recoveries increased and now each re-trap or ‘control’ of a live bird ringed by someone else adds another fact to that bird’s life history.
In addition to the story of migration and family and social relationship one also obtains much information about other facts of a bird’s life. The booklet Bird Ringing lists, for instance, the maximum age of a number of birds as shown by ringing recoveries. I have extracted some which seemed to me to be of particular interest.
Ringing has shown that young Fulmars leave their ledges when they are about seven weeks old. They spend the next three or four years at sea before they finally return to the cliffs to prospect for nest sites for several seasons. Only then do they begin to breed, in about their seventh year. Their mean expectation of life is about sixteen years, but they have a potential life span of fifty years. Dr David Lack calculated that the average further life (life after its first year) for an adult songbird is 1-2 years, for various waders and gulls 2-3 years, and for Swifts 4-5 years.
The ringing recovery of a species of which fairly large numbers have been ringed can give interesting information about life expectation, mortality rate, etc. The mortality rate of some birds is very high but is balanced by a high rate of breeding. Chris Mead in his booklet points out that if only one young survives from each pair’s breeding activity each year the Robin population would increase by more than 50 times every 10 years. Chris Mead also calculated the annual mortality for three common species: 35% for the Blackbird, 53% for the Starling and 44% for the House Sparrow. Two larger species for which figures have been worked out are 15% for the Herring Gull and 6% annual mortality for the breeding population of Fulmars in Scotland.
Some people worry that ringing or the actual wearing of a ring is cruel to birds and that they might be damaged when they are being caught or that the weight of the ring will hinder their feeding. As I indicated earlier, every ringer in Britain has to undergo a strict training programme before he is allowed to ring on his own and while accidents can always happen most ringers now operate with a strong sense of their own responsibility. The rings are very light in weight and should fit fairly snugly; even when I used an aluminium ring and three plastic rings on Wheatears, the total combined weight was, relatively speaking, less than one shoe on a human being or less than the daily variation in a Wheatear’s body weight: the Wheatear puts on weight as it feeds during the day and loses it during the night. There really is, generally speaking, no evidence that ringing harms birds. Furthermore, the table of longevity shows that ringed birds are able to survive to a good age. In addition some of the knowledge gained which has been used in public enquiries has benefitted whole populations of birds.