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Swifts delayed by cold and wet springs face mounting problems as the climate changes

Swifts delayed by cold and wet springs face mounting problems as the climate changes

The weather is warmer and the nights are lighter. What are those black, curved silhouettes looping in the sky?

Assuming you are looking at birds and not attending the World Boomerang Championships, those shapes will likely be the UK’s only breeding member of the Apodiformes (a grouping that includes the hummingbirds): the common swift (Apus apus), harbinger of summer.

People in the UK tend to think of these birds as theirs, but really, they are tropical African species that spend a few months of the year at temperate latitudes. Competition for food, shelter and mates is intense in the tropics, so swifts evolved migration to take advantage of the seasonal boom in plant growth at temperate latitudes. Their breeding range stretches from Ireland to China and birds from the eastern edge of their range make epic annual round trips of 30,000km.

For swifts, migration to temperate latitudes should mean access to lots of flying insects to feed their young and much less competition from other species to eat them. Like other aerial insect feeders, swifts can’t stay through the winter as there is nothing to eat and so nothing to keep them here.

These birds are much-loved symbols of the changing seasons. So how are they faring as rising temperatures change when seasons arrive and what form they take?

Swifts getting swifter

Swifts are arriving on average about five days earlier in the UK than they were in the 1960s, but they are also leaving significantly earlier too – the only common migrant bird to do so in the UK. Many other species are deciding to extend their summer residency and have multiple broods.

There is still variation between years, but the swifts were very late this year. In this wet, cold spring, I saw my first here in the Longdendale Valley of Derbyshire in England on May 3; by way of comparison, in the unseasonably sunny weather of spring 2020, I saw my first Longdendale swift as early as April 19.

Cold and wet weather is bad for anything that eats flying insects, which aren’t active in these conditions and are more difficult to catch even if they were. One way swifts can weather these challenges is by going on very long foraging trips far from their nest sites to avoid storms. Adults and nestlings can also lower their metabolic rate while sitting on the nest (a phenomenon called torpor) to save energy.

Swifts struggle with prolonged wet periods. A recent study linked an increase in rainfall in June and July from 1975 to 2015 with higher nest failure rates, smaller broods and lower survival of swifts in their first year of life, as they can’t find enough food to feed themselves or their chicks. This is likely to be compounded by potential declines in their invertebrate prey, which are very sensitive to weather conditions like temperature and rainfall.

Cold, inclement springs like this year’s may also kill large numbers of swifts as they migrate through the Mediterranean by preventing them refuelling, leading to starvation, although there have also been instances of heatwaves killing large numbers of swifts as they overheat in their nests in roof cavities in the same region, something which might become a problem in the UK in future.

The cues of birds to migrate come both from an internal clock and factors in their environment, like changes in the seasons. Evidence suggests that species which are not adjusting their migratory schedules in the face of global change are more likely to be in decline and swifts do not appear to be as flexible as some other species in arriving earlier.

Tailored swift bricks

Longer-distance migrants are then more likely to be declining than shorter ones, but scientists’ understanding of the role of climate change in driving declines is still limited and complicated by so many other facets of global change.

Beyond reducing the carbon emissions and habitat destruction associated with our diets and lifestyles (and voting for politicians who take these challenges seriously), providing swift nest boxes may be another way people can help.

A campaign by Hannah Bourne-Taylor, a nature writer, to mandate the installation of “swift bricks” in new UK housing stock, could make up for the loss of nest sites in the eaves of older buildings – if only the government would support it.

Given all the difficulties swifts face and the joy that so many people derive in watching and listening to them careering across our summer skies, then we really should be supporting any efforts that may make a difference.

Alexander C. Lees, Reader, Manchester Metropolitan University

This article is republished from The Conversation under a Creative Commons license.