Tag project that’s far from cuckoo

THE cuckoo is in trouble.

Its numbers have fallen by 50% in the last 30 years and nobody really knows why.

Has something gone wrong with the environment here or does the source of the problem lie elsewhere?

This vocal bird is conspicuous in summer but, ironically, it drops off the radar once it leaves our shores; we know almost nothing about its life in Africa. Until we discover where the bird goes and the routes it takes getting there, we can’t fully address the problem.

More than 6,000 cuckoos have been ringed in Britain and Ireland, only 2% of them being seen again. Almost all foreign records so far are from mainland Europe. A bird found in Cameroon, 82 years ago, is the only one reported from sub-Saharan Africa, apart from a Dutch-ringed juvenile which went to Togo. Clearly, just putting rings on cuckoo’s legs and waiting for reports to come in isn’t going to answer any fundamental questions. Now, however, extraordinary new technology is being deployed and the British Trust for Ornithology (BTO) is confident that it can solve the riddle of cuckoo migration.

In 2011, BTO ringers fitted tracking devices to five cuckoos in Norfolk. The solar-powered ‘geo-locators’, weighing as much as a 20 cent coin, don’t hamper the birds.

They transmit for 10 hours, then ‘sleep’ for two days while their batteries are being recharged. The signals are picked up by Argos satellites orbiting overhead. When an ambulance passes by, the pitch of its siren falls; the Doppler Effect. As satellites pass over a transmitter, the frequency of the radio waves they receive alters in similar fashion, enabling the bird’s location to be determined to an accuracy of about 500 metres.

The results of the project to-date have been so extraordinary that 11 more cuckoos, including Scottish and Welsh ones, were given transmitters this summer. Two of the original five tagged birds have survived, bringing to 13 the number now being tracked.

It isn’t easy to catch cuckoos. In almost 30 years of ringing, I never handled one. The main trapping device is the mist-net, a huge spider’s web suspended between three-metre high poles.

Against the back-drop of a hedge or a reed-bed, the black nylon mesh is almost invisible to a bird. Cuckoos, however, seldom fly low enough to be caught in nets and they are very adept at escaping if they do encounter one. Skilled bird trappers recommend that nets with extra large meshes be placed around an isolated bush. Male cuckoos ‘sing’ from such vantage points and females use bushes as observation posts from which to watch the comings and goings of little birds in whose nests they will deposit their eggs. Playing recorded female song, with a stuffed female cuckoo mounted near the net, tricks males into coming close. With luck, they can be caught.

The BTO website (www.bto.org) has a map showing the migration routes taken to-date. The birds ringed last year stopped off in Europe for rest and recreation but, to everyone’s surprise, they chose widely differing routes on the journey down through Africa. Two of the birds headed for the west coast and travelled through Morocco Senegal and the Gambia. They have the distinction of being the first cuckoos ever recorded in these countries.

The other three birds took a shorter but more hazardous route, flying directly across the arid wastes of the Sahara. All five birds spent the winter in the Congo, less than 70km from each other, although, according to the BTO, some were up to 3,600km apart at times on their journeys south.

The information on the two survivors from last year is most complete because the dates of their return to Europe are known. One of them stayed six weeks in Britain this summer before heading south again.

All four of the birds ringed in Wales had moved to mainland Europe by June 21 but two of the Scottish birds were still at their ringing locations then.

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Saturday, September 24, 2022

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