BTO experts seek to solve migratory mystery of the house martin

House martins nest-building. Pic by Doug Welch/BTO

House martins nest-building. Pic by Doug Welch/BTO - Credit: Doug Welch/BTO

Norfolk-based bird experts have a launched a high-tech bid to solve one of ornithology's greatest mysteries... where on Earth do our house martins spend the winter?

The British Trust for Ornithology (BTO), based in Thetford, has lots of data on the breeding ecology of this enigmatic bird between April and September, when it nests under roof eaves in the UK.

But once it departs in search of warmer climes in the winter, it all but disappears from the radar.

The birds spend the winter in Africa, but it is not known exactly which part of the continent they head for, or precisely how they get there.

This summer, the BTO aims to answer these questions by fitting birds with a tiny 'geolocator', weighing less than a gramme and about the size of a shirt-button, which stores data from a clock, a calendar and a light sensor.

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By comparing daylight length measured by the light sensor with the time and date recorded, scientists at the BTO hope to determine exactly where the device was at any given time between when it was fitted and when it was recovered.

This information could then reveal the birds' wintering areas, precise migration routes, and possible stopover and 'refuelling' sites.

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Researchers hope the answers to these questions will help explain the decline of the house martin, whose breeding numbers in the UK have fallen by two-thirds in the last 25 years, leading to the species being 'amber listed' as a bird of conservation concern.

Paul Stancliffe of the BTO said: 'I have long dreamed of being able to follow a bird like the house martin on its migration from Britain to Africa, to get a glimpse of the places it is passing through and the places that it chooses to stay and rest for a while before continuing on its journey.

'It is very exciting to think that we are on the brink of new discoveries that should help these delightful birds and provide them with a more optimistic future.'

Anyone interested in helping the project can find out more by visiting the trust's website at

Mr Stancliffe said: 'This technology comes at a price and we need help to secure enough of them to make the project worthwhile.

'Each device costs £170 and we hope to be able to fit them to at least 20 birds. We need help to support the scientists developing this project.'

Despite many thousands of house martins being ringed in Britain each year, relatively few are observed in Africa. This has led to speculation that they might spend long periods feeding at high altitude, including above the rainforest, where they remain unnoticed.

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