Outdoors: Admiring the canny cuckoo

But now the time has come, for the nest already contains a couple of tiny dark mottled eggs. Carefully picking a moment when the female pipit has temporarily flitted her nest, the cuckoo swoops down in a flurry of wings and quickly gulps down one of the eggs. She then straddles the nest with half-open wings and lays one of her own as a replacement, before taking off into the air again.

It is all over in a matter of seconds and it won't be long before the meadow pipit returns to her nest, unaware that cradled in its intricately woven cup is a time bomb that once hatched will destroy the rest of her brood. As for the cuckoo, well, her work is not yet completed as she has several more eggs to deposit in the nests of more unsuspecting pipits.

Such plunder and skulduggery will be happening throughout our hills and glens over the next few weeks, with the first half of May heralding the main influx of cuckoos into Scotland from their African wintering grounds, which soon highlight their presence by their familiar cuckoo call.

But underlying that almost hypnotic call is a bird with a most unusual breeding strategy, in which unwitting foster parents do the hard work of incubation and rearing young. It is a tactic with obvious advantages because the cuckoo doesn't have to expend the energy raising a family. It also means the cuckoo can lay more eggs than would otherwise be the case if it were looking after its own.

But the ploy has inherent risks. If the cuckoo is too slow while laying its egg in the host nest, there is every chance that a watchful crow will witness the incident and make its own swoop to devour the contents.

Research in England, where the reed warbler is a common host, has shown that if the warblers detect a cuckoo in the vicinity, they are more likely to carefully check their eggs and reject any that look suspicious. The cuckoo, therefore, has to be stealthy in its work.

The cuckoo's egg, while slightly larger than that of the host, is otherwise a close mimic in terms of colouration. Different populations of cuckoos specialise in different foster species and have evolved to produce eggs that closely match the colouration of their hosts.

The meadow pipit is by far the most common species exploited in Scotland, although others such as the dunnock are occasionally targeted.

At only twelve and a half days, the incubation period of a cuckoo egg is shorter than that of its host, which means it is often the first in the clutch to hatch, giving the naked and blind chick a crucial advantage. During these first few days, when it feels the pressure of another egg or chick against its side, it instinctively arches its back to throw the load out.

Soon, only the baby cuckoo remains, with its unusually loud begging call hoodwinking the strong parental instincts of the fosterers into working around the clock to feed it until it reaches independence. This instinct is so great that the foster parents don't twig that something is wrong even once the youngster has grown much bigger than them.

In England, cuckoo numbers have suffered a significant decline in recent years, probably related to habitat loss and subsequent declines in its main host species.

The situation in Scotland is unclear, with some recent survey data showing there may have been an increase but other anecdotal evidence suggesting a decline. Here, they favour open upland areas and the abundant meadow pipit population should help sustain numbers, although the cuckoo is always vulnerable to fluctuations in its main food source of hairy caterpillars.

There is no doubt that the cuckoo is the ultimate vagabond – but our hills would certainly feel much emptier without its echoing call, one of the iconic sounds of late spring.

This article was first published in The Scotsman on Saturday, May 1, 2010