Whitethroats feeling the heat

The whitethroats numbers have been affected by climate change in its winter home in the Sahel.

Whitethroats feeling the heat

WHITETHROATS are nesting in a bushy hedgerow close to where I live. This might seem unremarkable but this little bird has not been seen in our neck of woods for over 30 years. The species has taken a hammering in recent decades so it’s a joy to have them back.

The whitethroat is a warbler, a tiny distant relative of the thrushes. A summer visitor from Africa, it arrives in April and departs in August. It’s easy to tell the little grey-brown songster from other small birds; the gleaming pale throat is visible even at a distance. There’s a white ring around the eye but you have to get close to the bird to see it. No shrinking violet, the male will perch conspicuously, and broadcast loudly, from the top of a bush. The song is distinctive; bursts of jerky jingle, a bit like a dunnock’s, but shorter, more forceful and frequently repeated.

Around 2,000 ringers catch almost a million birds in Britain and Ireland each year, including about 10,000 whitethroats. Rings carry serial numbers and the words ‘Inform BTO British Museum London SW7’. ‘BTO’ stands for ‘British Trust for Ornithology’, organisers of the ringing scheme. Anyone coming on a ringed bird will, it’s hoped, report the find. These returns provide invaluable data on migration, life expectancy, and causes of death, in birds.

Apart from the information derived from the rings, catching the birds is valuable in itself, as whitethroats demonstrated 45 years ago. Statisticians at the BTO analyse the ringing data at the end of each year, identifying population trends. A drop in the number of juvenile birds caught in the autumn, for example, would indicate that there had been a poor breeding season. The sample of the bird population handled by ringers is ‘statistically robust’ for most species, so the annual avian health-check is reliable.

In the summer of 1969 reports began to surface that there were few whitethroats about. Being ‘anecdotal’, they were not treated that seriously. When the end-of-year ringing totals were calculated, however, ornithologists were in for a shock; the number of whitethroats caught had been tiny. Data from the Common Bird Census scheme showed that numbers were down by 77% compared to the previous year.

Songbird numbers fluctuate. A severe winter can decimate our resident birds. Migration too has its hazards and birds flying long distances face a myriad of dangers. Such glitches are usually temporary; small birds have such high reproductive rates that they soon make good the numbers lost in disasters. The whitethroat population, it was thought, would recover. It might take a year or two but ‘happy days’ would soon be here again.

But the whitethroats failed to return. In the years following the crash, numbers continued to decline. By 1974, they had dropped by a further 40%. Clearly, this was no temporary run of bad luck. Something more ominous was responsible, but what? Another species provided a clue. Sand martin numbers had suffered a similar, though not as severe, decline. Like the whitethroat, sand martins are long-distant migrant from sub-Saharan Africa. Something in Africa must be killing the birds.

The Sahel is a vast semi-arid transit zone between the Sahara desert and the savannah to the south. It stretches from the Red Sea to the Atlantic Ocean. In successive years since 1968, the annual rains failed to materialise in the Sahel. Areas were turning into desert. Whitethroats winter in the Sahel region. Others venture further south but cross it on migration. Victims of global warming, few of them could survive the harsh new conditions in the Sahel. They were not alone. Droughts continue to plague the region causing a humanitarian disaster. By the mid-1980s famine had killed 100,000 people, precipitated wars and caused severe dislocation.

According to the Bird Atlas 2007-11, ‘the British and Irish (whitethroat) population has never fully recovered from this crash’. There has been some improvement however. Numbers in Ireland seem to have risen by about 15% since the collapse 45 years ago.

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