BBC Wildlife - UK (2020-08)

Great spotted cuckoos, like the more

famous common cuckoos, are brood

parasites that lay their eggs in the nests

of other species and then abandon their

progeny to be cared for by the foster

parents. Because host parents generally

lose part or all of their brood due to the

presence of a voracious cuckoo chick, they

clearly benefit from evolving defensive

strategies against these uninvited guests –

attacking adult cuckoos that loiter nearby or

ejecting alien eggs from their nests. But the

persistent parasites have counter-evolved

more and more sophisticated tricks – such

as producing eggs that better resemble

those of their target species – in a

continuous ‘evolutionary arms race’ that

has fascinated scientists for decades.

However, our crow female sits placidly on

her nest while being closely observed by the

great spotted cuckoo. When the crow finally

flies away to forage, the cuckoo immediately

seizes her chance. She flies to the unattended

nest, sits in it for few seconds then makes

a swift getaway. We run over and check the

contents of the nest – three crow eggs and

one, considerably smaller, cuckoo egg.

Making themselves at home

The cuckoo female will spend the rest of

the spring laying up to 26 eggs, distributed

among various nests – which is remarkable,

compared to the ‘normal’ clutch of five or

six eggs of the main host species. Then,

in late June to early July, she (and all her

conspecifics) will start the long journey

towards Africa, where she will spend the

winter, before returning in the spring.

The great spotted cuckoo’s main host is

usually the magpie, but crow nests in our

study area are heavily parasitised, too – up

to 70 per cent in some years. While the

young of common cuckoos will evict all

other eggs and chicks from the nest shortly

after hatching, great spotted cuckoo chicks

don’t. Instead, they throw themselves into

begging behaviour, to make sure they

hold the attention of their foster parents at

mealtimes. They also tend to hatch a few

days earlier than their nest-mates, gaining

an age advantage that makes them far more

competitive in obtaining food. In fact, the

majority of magpie chicks starve to death

when forced to compete with these cuckoos.

It is therefore not surprising that magpie

adults use a range of defensive strategies

against these freeloaders. Crows, however,

seem to completely lack such animosity.

Though some crow chicks (larger than the

parasite) can make it to fledgling, brood

reduction due to the presence of the

The great spotted

cuckoo is slightly

larger in size than its

better-known cousin,

the common (or

European) cuckoo.

The cuckoo female will spend the spring

laying up to 26 eggs in various nests.

Mike Lane/Getty