Ringing birds with unique metal bands has provided valuable information about bird migration patterns and population trends. Recovering ringed birds helps track breeding and migration routes, like a chiffchaff that was re-trapped in the same area a year later. Satellite tags on cuckoos revealed their wintering grounds in Africa and important stopover sites in Italy, information that can help understand the species' decline. Continued tracking through ringing and new technologies may help reverse the population drops threatening many species.

1. Ringing has played a crucial role in tracking
the migratory routes of birds. Ellie Mayhew
explains how can it be used to gain
information on the issues that threaten
species in decline.
T
he sight of a swallow (Hirundo rustica) is for many
people the sign of spring. These insectivorous birds
overwinter in South Africa to maintain access to
their food source. Until the 1800s, it was a common belief
that these hirundines spent the winter hibernating at the
bottom of ponds! However, the bird ringing scheme, run
by the British Trust for Ornithology (BTO), has supplied us
with a wealth of information on the population trends and
migration habits of many British bird species, allowing what
was once considered fact to become folklore.
Birds are caught using several different methods
depending on the target species, the most common being
mist netting. Some species are also ringed as pulli (young)
in the nest. A small metal ring, which is engraved with a
unique ID number, is fitted onto the bird’s leg using special
pliers, and the sex and age are determined, where possible;
this is recorded along with biometrics such as wing length
and weight. Information is gained from both initial ringing,
as population sizes can be estimated and breeding success
measured, and from recovery of ringed birds. The term
“recovery” includes any dead ringed birds found and birds
which are re-trapped by a ringer. Recoveries can provide
information on site fidelity, age and migration routes.
I caught a chiffchaff (Phylloscopus collybita) at a site
in Dorset on 3rd April, which had been ringed there the
previous year. Interestingly, British chiffchaffs have several
sub-populations that migrate from different African
countries to different parts of the UK, and each year the
same sub-population travels to the same area. This was
discovered as a result of ringing and is proved by the re-trap,
which is a clear demonstration of breeding philopatry.
There are significant movements of birds in the winter
months too. Thrushes such as redwings (Turdus iliacus)
and fieldfares (Turdus pilaris) migrate from Scandinavia if
food is scarce, and woodpigeons (Columba palumbus) can
be seen in their hundreds moving south to France. A very
recent recapture of a blue tit (Cyanistes caeruleus) wearing
a Lithuanian ring at Sandwich Bay Observatory proves that
birds may travel much further than we think, as blue tits are
usually considered to be quite a sedentary species. Ringing
is often so full of surprises!
Very little was known about the migratory patterns of
the common cuckoo (Cuculus canorus) until five males were
caught in Norfolk in 2011 and fitted with solar-powered
satellite tags. The aim of this project is to gain information
on the annual cycle of this species, which could then help to
investigate why these birds have worryingly declined by 63% in
England in the last 25 years. Although 6,765 cuckoos had been
ringed, as of end of 2014, only 138 have ever been recovered.
Therefore satellite tags were likely to provide us with more
information than conventional ringing has.
So far, we have learnt that the wintering grounds of British
cuckoos are in the Congo Rainforest, in Central Africa. The
speed of migration varies; spring migration takes only two
months, whereas autumn migration takes around four months.
This is because, upon their arrival in the UK, cuckoos are
under pressure to find a mate and ensure food will be plentiful
when their young hatch. Important stopover sites have been
identified, such as the watershed of the River Po in Italy, which
are vital stages in migration as it is where the birds fatten up
before they cross the Mediterranean and the Sahara. If these
stopover points are suffering a drought or food is not plentiful,
the cuckoos may not survive their subsequent journey, so these
sites are an important factor to consider when understanding
reasons for the cuckoo’s decline.
There is a lot we have yet to learn about this mysterious
bird, but thankfully our knowledge has improved as a result of
this project, and eventually we can strive to reverse the decline
of this iconic species, as well as the many other species also
under threat.
Progress of the cuckoos can be tracked online at www.bto.org/
science/migration/tracking-studies/cuckoo-tracking
Ellie Mayhew is a qualified bird ringer and studies MSc
Conservation Biology and Ecology at the University of Exeter.
Mapping
Migration
14
SUMMER | MIGRATION | CONSERVATION
Photos: Ellie Mayhew (top), Will Hawkes (bottom)