Hardly a month passes without a startling new map appearing on some website showing the impressive movements of a bird, as studied with the help of modern technology. Tracking tagged birds from satellites has now given us the ability to follow individuals from day to day on their migrations, wherever in the world they travel. From samples of such tagged individuals, we can also calculate mortality rates during different periods of the year or year-round. Not surprisingly, some people are now asking whether, with all the new methodology, old-style bird ringing is still necessary. In my view, the answer to this question is an emphatic ‘yes’. In this editorial, I shall first mention the various methods of marking and tracking birds, and then go on to explain why I think that bird ringing is still essential to the development of both bird science and bird conservation.
Bird ringing dates back to 1899, when Hans Christian Cornelius Mortensen, a Danish schoolmaster, produced the first metal bird rings carrying a unique number and an address, which he attached to birds in his home area. His innovation gave a way of turning anonymous birds into recognisable individuals, identifiable as such for the rest of their lives. Ringing spread quickly, and soon became a mainstay of migration studies worldwide, revealing previously unknown migration routes to an astonished world. Only much later, in the 1940s, did ring-recoveries come to be used to estimate the annual survival rates of birds. The initial method involved comparing, for particular species, the numbers of individuals ringed as chicks that were subsequently reported dead at different ages by members of the public. From the ratios of birds reported in their first to second year, second to third year, third to fourth year and so on, the annual survival of different age groups could be calculated. Since then, more sophisticated statistical methods have been developed which can make use of information from birds ringed at any age, whether they are reported dead or alive.
Large numbers of birds must usually be ringed in order to provide a small number of recoveries, but the average recovery rate for birds ringed in Britain of 2% belies great variation between species and regions. Most small birds have recovery rates of less than 1% (excluding personal recaptures by the ringers themselves), but some larger birds can yield recovery rates of 20% or more, especially if they are hunted. In addition, recovery rates can vary markedly along migration routes, and it is generally difficult to get recoveries from tropical wintering areas. Many records are needed to provide a worthwhile picture of the migration routes or survival rates of particular species. The situation can be improved by use of coloured rings or tags that can be read in the field from live birds without recapturing them, providing repeated records from the same individuals. But, as with ring-recoveries, reporting rates can also be biased geographically.
Following ringing, another breakthrough in the study of bird movements was the development in the 1960s of small radio transmitters that could be fixed to birds, revealing their locations over distances of up to a few kilometres. Radio tags can be attached using a harness or fixed to a ring or a tail feather, and require an observer with a receiver and antenna to detect the signals. Now available in weights down to 0.2 g, such tags are used mainly to follow birds and other animals around small areas, such as their breeding territories.
From the mid 1980s, tags became available that transmitted automatically to satellites. By continually circling the globe, Argos satellites can detect signals from anywhere below, and then transmit the locations to a ground station. This technology enables birds to be monitored day by day on their journeys from breeding to wintering areas and back again, anywhere in the world. It provides almost real-time information on migration routes and timing, stopover locations and durations, flight speeds, wind and weather effects, and orientation abilities. For some species, previously unknown breeding or wintering areas have been detected, and some amazing journeys have been revealed (notably the non-stop trans-Pacific flights of godwits from Alaska to New Zealand). The earliest suitable transmitters for use on birds (called platform transmitter terminals, or PTTs) weighed more than 100 g so they could be used only on large birds, but development continued and, using solar power, PTTs are now available down to 5 g, with even smaller ones predicted for the future. They are currently used by BTO researchers to track Common Cuckoos Cuculus canorus to their African wintering areas(see www.bto.org/cuckoos). Tags are also now available which combine satellite with Global Positioning System (GPS) technology to give yet more precise location estimates than earlier tags.
The main drawbacks of satellite-based methods are the high costs of the units and the subsequent data processing by the service provider (up to about £3,000 per bird per year). The weight of the tags also means that they have so far been used chiefly on larger bird species, such as raptors, waterfowl and seabirds. Ideally, such units should weigh no more than 3% of the bird’s weight – less than a meal – in order not to affect the bird’s migratory behaviour significantly.
Cheaper, and lighter, types of ‘geolocator’ tags have been developed over the last decade, which store the collected data on board. One type (GLS) uses a photo-sensor and takes repeated measurements of the ambient light intensity so that the time of local sunrise and sunset can be calculated. These data allow us to estimate the approximate geographical position of the bird at different dates (latitude from daylength and date, and longitude from times of dawn and dusk relative to Greenwich Mean Time). Because they do not transmit information, such devices need only a small battery and can thus weigh less than 1 g; the big drawback, however, is that the bird needs to be recaught to retrieve the data. The units are inexpensive (around £100), but the need to recatch the wearers means that some tags are never recovered, which increases overall costs. Another drawback is that, because of the gradual onset of dawn and variation in cloud cover, location reliability is less than with satellite tags (some estimates can be in error by more than 200 km), but this may be acceptable in tracking long-distance migrations. GLS tags were used by BTO researchers to map the migration routes of Common Nightingales Luscinia megarhynchos.
Recently a second, more expensive, type of geolocator has become available which uses an on-board GPS to obtain accurate locations from satellites at pre-set intervals. Although the bird usually has to be recaught to retrieve the information, in some situations the data can be downloaded via VHF or the mobile telephone network if the bird comes within range of a base station. Initially, the weight of these tags (5-10 g) meant that they could be fixed only to larger birds. They have recently been used to map seabird feeding areas around Britain, providing information for the designation of Marine Protected Areas or the licensing of offshore windfarms. The latest devices weigh 1-2 g but then provide only a limited number of locations. Because the locations determined by GPS are accurate to within 10 m, the method can be used to gain precise assessments of a bird’s home range at different seasons, as well as its migration routes and stopping sites. Used in conjunction with high-powered satellite images or aerial photographs (such as those available from Google Earth), a bird tracked using GPS can be placed accurately within a landscape thousands of kilometres from the researcher. These various methods provide a much more complete picture of bird migratory and ranging behaviour than could ever be obtained from ringing. They have revolutionised the study of bird migration.
Owing to the costs involved, these new methods are normally used only in funded research programmes, involving small numbers of birds, from a limited range of species, over a limited number of localities and years. Ringing is also not cheap, considering that only a small proportion of the rings applied produce recoveries. But traditionally the cost of ringing in Britain has been borne (for the most part willingly) mainly by the ringers themselves. It is hard to imagine that such a massive database could have been accumulated over the years in any other way. Ringing can be applied to species of any size, year after year, on a large spatial scale (nationwide or greater), with the results continually added to an ever-growing long-term database. Moreover, the biometrics collected when birds are handled in large numbers provide valuable insight into other aspects of bird biology, such as breeding and moult, body weights, age and sex ratios, and even the incidence of disease.
Ringing data have shown their value in recent decades, when many bird populations declined, mainly though human impacts of one sort or another, and some species have also changed their migration patterns. If we are to detect and understand these changes, and take effective conservation measures, we need appropriate data. When birds decline, it is helpful to know whether reduced survival or reduced reproductive output is involved. For example, in most small seed-eating species in Britain, population declines in recent decades were associated with reduced survival, whereas in some other species survival stayed the same over a period of decline, suggesting that reduced reproduction was involved (Newton 2013). This knowledge in turn indicates where conservation measures should be directed, in breeding or wintering areas.
Even if ring-recoveries are insufficient for annual assessments, they can be pooled into blocks of years, comprising, for example, periods of population stability or increase and periods of decline. This type of retrospective analysis applied to farmland birds helped to identify the causal factors involved in different species, and is now being applied to some of our declining migrants. Such analyses can come only from large-scale, long-term data, such as those provided by ringing schemes. Another advantage of ringing, as it is now organised in Britain, is that it can cover all species all the time, not just those of current interest, and we can never know which ones will be in trouble in future.
Ongoing ringing is like having money in the bank, ready when needed. Only the presence of individuals that are already ringed in populations enables us to assess, for example, the provenance of seabirds killed in winter wrecks, the spread and impact of new diseases such as trichomonosis, or the risk of alien diseases (such as the H5N1 strain of avian influenza) reaching Britain.
Some bird species are becoming more sedentary than in the past, or shortening their migrations to winter at higher latitudes than previously, presumably in association with climate change. It would have been difficult, in the absence of ringing, to have recorded most of these changes. Nor would it have been discovered that the Blackcaps Sylvia atricapilla now wintering in small numbers in Britain are not drawn from among the birds that breed here, but derive from a different population, breeding nearer to central Europe. In view of the continual changes in our avifauna, I would argue that cost-effective data from centralised ringing schemes are just as necessary now as they always were to detect and understand these changes, enabling in some cases appropriate conservation measures to be taken.
People are often dazzled by technological innovations and tend to overlook the less charismatic new analytical techniques that are allowing much better use of ringing results to help explain population trends in demographic terms. The most recent BTO studies combine data from ringing, nest records and counting schemes to produce demographic models of bird populations that give unprecedented detail on large-scale population dynamics (Robinson et al. 2014). Similarly, the development of more systematic and standardised site-specific ringing-recapture programmes is allowing much more powerful analyses of year-to-year survival and population changes. New technology has not replaced ringing but it has greatly expanded our toolkit. It is the combination of methodologies that has given such exciting recent developments in our understanding of bird populations and movements, and helped so much in bird conservation.
Newton, I. 2013. Bird Populations. Collins, London.
Robinson, R. A., Morrison, C. A., & Baillie, S. R. 2014. Integrating demographic data: towards a framework for monitoring wildlife populations at large spatial scales. Methods in Ecology and Evolution: doi:10.1111/2041-210x.12204
For ease in reporting ring-recoveries, visit www.ring.ac