Mapping bird populations

Juvenile Common Teal by Steve Young (www.birdsonfilm.com)
Juvenile Common Teal by Steve Young (www.birdsonfilm.com)

The unfolding of the bird flu story earlier this year has highlighted how much still needs to be learned about bird movements if we are to understand the threat the disease may pose in the future. The development of a migration mapping tool by the British Trust for Ornithology will help us to find answers, and already it is showing the value of long-term surveys. Here, as an example, we look at how this new development helps to improve our understanding of the movements of a widespread species such as Common Teal.

Bird flu occurs naturally in many birds, usually in a relatively harmless way, but in 2003 a highly pathogenic strain, H5N1, surfaced in the Far East. In 2005 it spread westward and outbreaks were subsequently reported in Europe. The spread of the strain hit the headlines in 2006 and again in February this year, when H5N1 was found at a turkey farm in Suffolk. Although the poultry and cagebird trade is likely to have been involved, it was also thought that the cases in Western Europe in late winter 2005-06 pointed towards its spread by wild birds following cold weather influxes of birds from further east.

We need to know exactly where these birds are coming from and where they are migrating through. To find out in detail is a formidable task, and the BTO recently received funding from DEFRA to develop a ‘migration mapping tool’ that analyses the recoveries of ringed birds to determine the pattern and timing of migrations. This will help to assess the likelihood of bird flu coming to Britain if there is an outbreak in a species in a particular area elsewhere in Europe.


How was this achieved? Taking Common Teal as an example, we had more than 15,000 reports of ringed birds to look at and analyse. These have come from almost 100 years of bird ringing and are an excellent example of how information from ringing continues to contribute to our increased understanding of bird movements. We all know that we see more teal in winter, but these ringing recoveries can show us how these birds are distributed and how they move around Europe and further afield.

One way to look at all these recoveries is to map where they occur in spring, summer, autumn and winter. Using GIS (Geographic Information Systems) software, recoveries can be separated by season and then plotted on maps. Using ‘kernel mapping’ (originally developed in the United States to plot the home ranges of radio-tagged fish) we can identify where most recoveries come from. The maps show not only where birds occur in each season, but also where the main concentrations are. They also show clearly that the main movement of Common Teal is through Scandinavia and into Russia, with autumn and spring concentrations around the southern areas of the North Sea. However, the true pattern is perhaps not so simplistic, as we need to understand how the different populations are using Britain.

Mapping movements

Again with reference to Common Teal, by looking at the distance moved between ringing and finding as well as the direction of movement (Figure 1) it is possible to tell immediately that there are movements in many directions to and from Britain. These movements fall into two main and two smaller groups, corresponding to different populations. After plotting the distribution of these different groups back onto a map, it is possible to calculate the average location of birds in each month. So for Common Teal we can see four general patterns of movement.

The first pattern, is a broad-fronted migration of birds, with most arriving from Scandinavia, although some come from as far south as the northern Mediterranean. This is a post-breeding movement and shows that birds from this population are in Britain from October to February, starting to move back in March and reaching the breeding grounds in May and June. Return migration commences in July, and by September most birds are back around the North Sea.

The second movement is a redistribution of Common Teal from Britain, with birds flying southwards to the Atlantic coasts of France and Spain. It includes birds switching wintering areas between years (ie one year they winter in Britain and the next they winter in Spain, so it appears they have moved from one to the other) and birds moving away from places hit by a very cold spell, as well as movements involving birds breeding in southern Europe.

The other two patterns involve fewer birds. Of these, the first is a relatively local east to west movement of birds between Britain and Ireland and the second is the migration of Icelandic breeding birds (Figure 4). These Icelandic birds tend to leave later and arrive earlier in the year than birds that breed in eastern Eurasia.

Using these mapping techniques, we now have a good idea of what various different species of waterfowl are doing through the year. So if an outbreak of bird flu occurs on the Continent, we can pinpoint which of the species in that area at that time might migrate to Britain.

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The importance of ringing

The development of this mapping tool highlights the real importance of ringing data in understanding bird movements, as well as the value of the dedication and hard work of the many ringers involved in the BTO scheme. It also underscores the fact that we do still need to ring large numbers of birds in order to generate the recoveries that allow us to do this kind of work. And it is in the recovery of data that everyone can make a contribution – the next time you see a ringed (or colour-ringed) bird on your local patch or find one dead on the shore, be sure to report it (the best way to do so is via www.ring.ac).