Migration and the decline of the cuckoo

Saving the cuckoo requires scientists to look beyond the borders of the countries within which it breeds. Breeding cuckoos in the UK have declined by 70 per cent over the past 25 years, but only in the past few years, thanks to the efforts of the British Trust for Ornithology (BTO), has it been discovered that UK cuckoos mainly winter in and around the Congo rainforest and Gabon. The routes they take to reach the Congo are also increasingly understood and these highlight some of the threats the birds face. Satellitetagged cuckoos followed by BTO researchers use two different routes from the UK to reach their wintering grounds: one through Spain and a second through

Italy and the Balkans; all of the birds make their spring return via the western route.

Birds that used the western route through Spain faced higher levels of mortality during the autumn migration. The BTO believes that climate change and changing land use in southern Spain may be having an impact on these birds, reducing foraging opportunities and the abundance of the hairy caterpillars on which they feed. These caterpillars fuel the cuckoos’ migration and if a bird can’t find sufficient food in these stopover areas, then crossing the Sahara becomes perilous.

Land-use pressures in sub-Saharan Africa are also affecting the ability of birds to forage and thrive during the northern winter. Pressures include the clearing of forest to create space for oil palm plantations. This change in land use appears set to accelerate.