Weatherwatch: climate shifts threaten birds’ return as spring arrives

As birds begin long journey north, climate-driven seasonal changes may leave late arrivals struggling to find food for young

Thursday is the spring equinox, when day and night are more or less equal all over the world. For naturalists, it marks the official start of spring, though judging by the birdsong in my Somerset garden, the season began several weeks ago.

As we eagerly await the return of swifts, swallows, warblers and flycatchers – all long-distance migrants from sub-Saharan Africa – we should reflect on how shifts in the world’s climate are causing them problems.

Their incredible journeys are governed by the tiny changes in daylight at this time of year. These shifts trigger the urge to migrate – what the Germans call Zugunruhe, a kind of migratory restlessness – which spurs them to head north.

These changes in light are constant, unaffected by changes in the climate. But when the birds return to our shores, at the same time as they always have, the seasonal shifts caused by the climate crisis mean that spring is already well under way. As a result, the insects on which they feed their young may already have peaked in numbers.

But not all migrants leave Africa and arrive in Europe at exactly the same time. This means that any early birds might now have an evolutionary advantage over their later relatives. Getting back a week or two before they usually do might allow them to synchronise their breeding with earlier springs, and raise a brood while food is still available. Latecomers, however, will lose out.

Cover photo:  Migratory birds’ journey north is triggered by tiny changes in daylight. Photograph: Jim Gibson/Alamy