Cuckoos are medium-sized birds, widely recognized for their distinctive call and a breeding strategy that involves laying their eggs in the nests of other species. This practice, known as brood parasitism, sets their lives apart from most other avian species. Their food choices are highly specialized compared to the general insect diet consumed by many common birds, allowing them to exploit resources others cannot access.
The Core Diet of Adult Cuckoos
Adult cuckoos are predominantly insectivorous, spending time foraging in foliage and on the ground for common invertebrates. Their general diet includes a variety of insects that supplement their specialized meals, ensuring a reliable food source throughout their active season. Typical prey items consist of grasshoppers, beetles, flies, and spiders, which they glean from leaves and branches.
When insect populations are low or during specific seasons, cuckoos broaden their diet to include other small creatures. This may involve capturing small lizards or frogs encountered while hunting on the ground. Some cuckoo species also consume small wild fruits, such as elderberries and wild grapes, especially as summer transitions into autumn or during their non-breeding wintering periods.
Specialized Feeding Habits
The defining characteristic of the adult cuckoo diet is their preference for hairy caterpillars, which are toxic or distasteful to most other birds. These caterpillars, such as those from the Magpie Moth or Drinker Moth, possess irritating spines or chemical toxins that act as a defense mechanism. The cuckoo has evolved specific physical and behavioral mechanisms to safely consume this challenging prey.
Before swallowing, the cuckoo often vigorously rubs the caterpillar against a hard surface like a branch, physically stripping away many defensive hairs. They may also squeeze the caterpillar to expel its gut contents, which can contain toxic plant material. Hairs that are consumed become embedded in the thick mucous lining of the gizzard, a muscular part of the stomach.
Unlike other birds that would suffer digestive tract irritation, the cuckoo’s stomach lining, complete with the embedded spines, is periodically shed. The bird then regurgitates this lining, forming a pellet that effectively cleanses its digestive system. This unique adaptation allows cuckoos to monopolize a plentiful food source that other insectivorous birds avoid, requiring an average cuckoo to consume hundreds of these caterpillars daily during peak season.
Diet of Cuckoo Nestlings
The diet of a cuckoo nestling is fundamentally different from that of an adult because it is raised by an unrelated host species. The young cuckoo is entirely dependent on the foster parents, such as dunnocks, meadow pipits, or reed warblers, for its nourishment. The food it receives is whatever the host species normally collects for its own young.
This host-provided diet typically consists of a broad range of soft-bodied insects and larvae, which are easier for small chicks to digest. The young cuckoo receives a continuous supply of these common insects, not the specialized caterpillars the adult seeks out. The cuckoo chick grows rapidly, often becoming three times the size of its foster parent, by monopolizing the entire food supply intended for the host’s original brood.