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.
References
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
Ian Newton
For ease in reporting ring-recoveries,
visit www.ring.ac
