91 : The Migration Atlas

2002

Surveys being undertaken using a mist net (above left) and Heligoland trap (above centre). The wealth of information on birds’ international and global movements provided by ringing recoveries has been an ongoing revelation, and the most important and interesting discoveries were documented in The Migration Atlas.

Where do birds go? It’s a question which has perplexed Man for millennia, long before the concept of migration was understood. While bird movements were widely observed and occasionally remarked upon in early literature, myths prevailed and a few remained popular even as recently as the 19th century. Swallows, for example, were sometimes believed to hibernate at the bottom of lakes, doubtless because of their habit of roosting in reedbeds in large numbers in autumn before suddenly vanishing. A published account from the same era purported to describe three living but dormant Corncrakes excavated from a dung heap in Monaghan, Ireland.

The dispelling of such fanciful beliefs required hard evidence, something which finally became possible with the advent of ringing (see pages 82–83). Ringing has remained the essential method of directly monitoring the movements of birds for more than a century since, but with a recovery rate of no more than 0.18 per cent at its highest, huge numbers of birds must be caught and ringed for meaningful knowledge to develop.

Early migration studies can attribute their success to a tradition of catching migrant songbirds for the pot. On the German North Sea island of Heligoland, migrant thrushes were trapped for food using troosel-goards, or ‘thrush bushes’ – spaces about six metres long by two metres wide, surrounded by a wall of bushes placed close enough together to let the birds only enter at the bottom, with a net over the top and another on the ground itself; birds were then scared to the centre of the bushes, becoming entangled in the net.

This ingenious method was adapted for bird ringing as the Heligoland trap in 1919 or 1920 by Dr Hugo Weigold, using a more funnel-like arrangement leading to a walled wooden trap at one end. The first British Heligoland trap was erected on Skokholm, Pembrokeshire, in 1933, and this means of trapping has since been used to great effect by bird observatories and migration hot-spots from Shetland to Scilly. More portable options later came in the form of fine-gauge nylon mist nets, no heavier than a mosquito net, and ideal for use in the field. In time, the volume of trapped birds, and thus ringing recoveries, grew to a substantial level.

Ultimately, however, the immense amounts of data collected over more than a century had to be fully analysed to draw useful conclusions about the movements and changes in migrant bird populations. Countless scientific papers have derived from ringing activities, but the greatest mainstream summary of knowledge is The Migration Atlas, published by the British Trust for Ornithology through T & AD Poyser in 2002 from the vast amounts of information collected since ringing started in 1909.

Data from more than half a million recoveries of 188 bird species in Britain and Ireland were set out in full and a further 73 species ringed in smaller number were featured in shorter accounts. The maps in the hefty but engrossing tome were the focal point for many birders, visualising epic journeys and feats of nature determined only through those decades of ringing study: Welsh Manx Shearwaters reaching south-east Brazil; a Stone-curlew ringed in southern England and recovered in Mallorca; Arctic Terns in southernmost Africa; even migrant Blackbird, Ring Ouzel, Redwing and Song Thrush on Heligoland. The Migration Atlas illuminated such movements for scientist and birder alike in unprecedented detail, demonstrating the value of the practice of ringing.

More than a century after ringing commenced, it clearly remains an important tool in our understanding of the movements of migrant birds. But innovative micro-tracking and tagging techniques are now filling in the gaps with real time continuous movement data (see pages 152–153), again partly due to the persistence and curiosity of the BTO.