The Common cuckoo is a medium-sized slender bird found throughout Europe and Asia. It has a greyish body and long tail, similar to a sparrowhawk in flight. There is a rufous color morph, which occurs occasionally in adult females but more often in juveniles. Common cuckoos in their first autumn have variable plumage. Some have strongly-barred chestnut-brown upperparts, while others are plain grey. Rufous-brown birds have heavily barred upperparts with some feathers edged with creamy-white. All have whitish edges to the upper wing-coverts and primaries (flight feathers). The secondaries and greater coverts have chestnut bars or spots. The most obvious identification features of juvenile Common cuckoos are the white nape patch and white feather fringes.
Common cuckoos are widespread summer migrants to Europe and Asia, and winter in Africa. These birds inhabit forests and woodlands, grasslands, meadows, shrubland, heathland, reedbeds, open moorlands, and cultivated areas.
Common cuckoos are generally shy birds that lead a solitary lifestyle; however, during the breeding season, they become noisy and are often heard singing their far-carrying song while claiming their territories and attracting mates. The male's song sounds like 'goo-ko' and is usually given from an open perch. The female has a loud bubbling call. Common cuckoos are active during the day spending many hours in search of food. They are especially fond of noxious hairy types of caterpillars typically avoided by other birds. Cuckoos are unusual among birds in processing their prey prior to swallowing; they will rub it back and forth on hard objects such as branches and then crush it with special bony plates in the back of the mouth.
Common cuckoos start singing early in the year in April. The male typically sings with intervals of 1-1.5 seconds, in groups of 10-20 with a rest of a few seconds between groups. The wings are drooped when calling intensely and when in the vicinity of a potential female, the male often wags its tail from side to side or the body may pivot from side to side. Common cuckoos are brood parasites which means that they don't build nests but lay their eggs in the nests of other birds. A female may visit up to 50 nests during a breeding season. She will remove one of the host's eggs from the nest and then lay her own. The eggs of Common cuckoos are spotted or solid in color, depending upon the color of the host's egg. The naked, altricial cuckoo chick hatches after 11-13 days. It is a much larger bird than its hosts and needs to monopolize the food supplied by the parents. The chick will roll the other eggs out of the nest by pushing them with its back over the edge. If the host's eggs hatch before the cuckoo's, the cuckoo chick will push the other chicks out of the nest in a similar way. The Common cuckoo chick usually leaves the nest 2-3 weeks after hatching and will first breed at the age of 2 years.
Common cuckoos don't face any major threats at present.
According to the IUCN Red List, the total population size of the Common cuckoo is 40,000,000-74,999,999 mature individuals. The European population of the species consists of 5,960,000-10,800,000 breeding males, which equates to 11,900,000-21,500,000 mature individuals. Currently, the Common cuckoo is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.