The distinctive call of the Cuckoo has been heard at RSPB Geltsdale. A dove-sized bird with a blue-grey back, head and chest and dark barred and white underparts, a bright yellow ring around their eyes, yellow feet and a black beak, the Cuckoo is a summer visitor to the UK and is known for laying its eggs in other birds’ nests. Instead of building their own nests, Cuckoos are ‘brood parasites’. They move into the nests of birds such as Meadow Pipits, Dunnocks and Reed Warblers, removing one of their eggs while the host birds are away and laying their own in its place. When the young Cuckoos hatch (after just 12 days), they often push the host’s eggs or chicks out of the nest and then eat all of the food brought by the host bird themselves. By the time the baby Cuckoos leave the nest, they have grown two or three times bigger than the host bird, but the host still continues to feed them for two weeks after they fledge.
Adult Cuckoos are able to leave the UK to migrate back to Africa much earlier than most birds, as their young are being raised by other birds. Most depart in June, with the young leaving a few weeks later, once they have fully fledged. Cuckoos can therefore be seen in the UK from April to July, and are found in woodland, especially around the edges, as well as on moorland and around reed beds. They feed on invertebrates, favouring hairy caterpillars in particular – and actually eat poisonous caterpillars that most birds avoid, shaking out the toxic substance inside them before swallowing them! They hunt for their food in bushes and trees, and are diurnal, meaning they are active during the day. The best way to identify a Cuckoo is to listen for it, as the tell-tale ‘cuck-oo’ call which gives the bird its name is easily recognisable. Females also make a distinctive, bubbling call. The birds congregate in areas where large numbers of Meadow Pipits, Dunnocks and Reed Warblers are found, so if you have seen these species around, it’s worth listening out for the Cuckoo’s call. They are often seen perched very still, looking out for caterpillars or unattended nests. In flight, Cuckoos look similar in size and colour to a Sparrowhawk. The easiest way to tell them apart is by looking at the tail, as Cuckoos have a graduated tail. Female Cuckoos can be rusty brown in colour on top, rather than grey like the males. Juveniles are brown on top, with heavy, black bars and white-tipped feathers. Their underparts are white with fine, dark grey bars and they have a white patch on their nape.
The numbers of Cuckoos have declined in recent years, due to a combination of loss of habitat and the knock-on effects on their host species in the UK, and deforestation and hunting on their migration routes. As a result, they are now classified as a Red List species in the 2021 UK Conservation Status Report, which indicates that they are of the highest conservation priority.