For the care and feeding of its offspring, the common cuckoo outsources.
When she’s ready to lay an egg, a female Cuculus canorus swoops to the unattended nest of a smaller species. She then swallows one of the eggs that’s been laid there and lays one of her own—a behaviour known as brood parasitism.
Sometimes potential victims revolt. The parents that inhabit the nest may mob the cuckoo mom, preventing her from dropping off her egg; they may push out cuckoo eggs before they hatch, or they may even abandon the nest.
But often the cuckoo mom gets away undetected, leaving her parental duties behind, and the nest’s owners return none the wiser. C. canorus is known to have passed its eggs on to more than 100 host species, according to the International Union for Conservation of Nature.
When the cuckoo chick hatches, it ejects other eggs or hatchlings to get all the space—and food—for itself. Hungry as a whole brood, the outsize baby devours everything brought by its foster parents—in the photo above, the provider is a reed warbler, a common host.
Franka Slothouber, a retired photo editor who’s an avid wildlife photographer, observed the birds’ behaviour in 2014 in Amsterdam, where she lives. “The poor warbler almost disappears in the wide-opened mouth of its ‘adopted’ baby,” Slothouber says. And yet “the warbler couple is convinced this chick is theirs and treat it accordingly, by feeding it until it can look after itself.”
This story appears in the January 2018 issue of National Geographic magazine.
Lead Image: A reed warbler (left) feeds a cuckoo chick. In 30 days cuckoo chicks grow from 28 grams to 90 grams—nearly eight times the warbler’s weight. PHOTOGRAPH BY FRANKA SLOTHOUBER