Cuckoos and Wrens by Sebastian Seely

An amazing series of photos taken by Peter Allman at the National Trust's Calke Abbey in Derbyshire caused quite a stir on social media last week. They show a juvenile Cuckoo being fed by a Wren (below). Lots of people got in touch to ask about this behaviour and about the behaviour of Cuckoos generally. Young birder Sebastian Seely has been here on work experience this week and spent a few hours in the BTO library researching answers to these questions...

Photo by Peter Allman

It has been known for birds to feed juveniles of another species even if they did not raise them themselves, as they are attracted to the sound of a hungry chick and have a strong instinct to feed them. However, in this particular case, the cuckoo has been observed being fed by the wren numerous times, so it is far more likely that it was raised by the wren in its nest. This is a relatively common occurrence in other European countries, however this is a relatively rare occurrence in Britain, there are only three or four records of it. This is probably most likely due to the fact that a wren nest isn’t really an ideal place to nest in as it is very small, and close to the ground. It could also be because wren nests are generally well hidden and hard to find, as well as being dark inside as a result of its dome-shape, making it more difficult to see and record cuckoo chicks.

How the cuckoo lays the egg in the wren’s nest is very much the same as they do for most bird species’ nests; the cuckoo simply clings to the nest, places her cloaca against the nest hole and squirts the egg in. The egg will then slide down inside the dome-shaped nest so it is quite unlikely that the egg will break. Sometimes the female cuckoo will take one of the wren eggs out with her beak, to make room for her own or just to eat as a snack.

The wren will probably not realise the cuckoo egg is not one of its own, and if it does, a wren will probably not try to reject it, because with rejection comes cost. For example, because the wren has such a small beak, it will find it very difficult to remove the egg so it may fall on its own eggs, smashing them, leaving it with no achievement at all. 

Cuckoo chick in a Reed Warbler nest. Photo by Kevin Carlson

As the cuckoo egg hatches, the chick will push the wren eggs, one at a time onto a curvature in its spine and push them out of the nest hole. The specially constructed wren nest makes it quite difficult for the cuckoo chick to eject the eggs so if it does not succeed in removing them all, it will outcompete the other wren chicks when they hatch as the cuckoo will grow into a much bigger bird very quickly.

Cuckoo chick being fed by a Dunnock. Photo by Derek Belsey

These photos also prompted many questions regarding why the cuckoo does not imprint on the wren as it is the first thing it sees, the same way a duckling can imprint on a human. The cuckoo may well spend the rest of its life thinking it’s mother is a wren, but it does not matter because after a few weeks it will become independent and most likely instinct will tell it to migrate to Africa, to eat caterpillars and to recognise and mate with other cuckoos. Some female cuckoos are known to lay their eggs in the same species’ nest as they were brought up in, suggesting that they do “remember”, their host parent or maybe just the habitat it was found in. Perhaps it’s a bit of both.

Sebastian Seely