The Coll of the Cuckoo

There are few birds with as much symbolism attached to them as the common Cuckoo (Cuculus canorus). In Japan it is a symbol of love whilst in Christianity it was a symbol of adultery, for ancient Greeks they were sacred and in Germany they spawned a whole new type of clock that has delighted and infuriated in equal measure for over 300 years.

Today in Scotland, the Cuckoo is seen as a sign of spring with most birds returning to their upland haunts by the end of April with the monotonous, onomatopoeic call echoing across the hills until mid June.

The Cuckoo is one of a select group of birds that is currently doing well in Scotland with the 2022 BTO Bird Breeding Survey report recording a 59% increase in the population over the last 25 years. Conversely the population in England has declined by a staggering 71% over the same period, so what’s going on?

Cuckoo’s famously lay their eggs in other bird’s nests with Meadow Pipits, Reed Warblers and Dunnocks being their preferred hosts across Britain. Laying your eggs in another bird’s nest is not easy and whilst the Cuckoo has evolved many strategies to help in its deception, the hosts have evolved just as many counter strategies to prevent the Cuckoo achieving its goal. By necessity, Cuckoos have to play the odds laying many more eggs than a bird their size would do normally and as such they require robust populations of their hosts. They also require food to fuel this egg production, and this takes the form of large, hairy moth caterpillars, just the type of caterpillars that have undergone severe declines in abundance (Garden Tiger moths have declined in abundance by 88% since the 1970s). It is likely that the change in population of Cuckoos across the UK is driven by a combination of these two factors, host populations and food availability with added pressures from climate change, but how are they fairing on RSPB reserves?

I have always had a fascination with Cuckoos so starting to answer this question as part of my sabbatical seemed like a perfect opportunity.

I completed my sabbatical on the RSPB Scotland nature reserve on Coll. This is an amazing place, home to Corncrakes, Hen Harriers, breeding waders and lots of Meadow Pipits and hairy caterpillars, paradise for Cuckoos. Male Cuckoos usually arrive first on the breeding grounds and can be heard calling and establishing territories from mid-late April. By the time I arrived at the end of May, territories were well established, and females had started finding Meadow Pipit nests and laying their eggs.

 Meadow Pipit perched on a fence post. Ben Andrew

Surveying Cuckoos involves selecting areas of suitable habitat and playing Cuckoo calls in the centre of a 1km grid square for 10mins and recording how many birds are attracted to the call. This method takes advantage of the bird’s territoriality but spaces out the surveys so as not to double count the same bird in the next square.

Using this methodology Cuckoos were recorded in 9 of the 12 surveyed squares with 11 birds recorded in total. In addition to this time was spent simply observing birds across the 9 occupied squares which was necessary to record the much shyer female birds who rarely responded to recorded calls. In total 8 females were recorded and by observing their movements and behaviours, areas of greatest risk of parasitisation were identified which led to a Cuckoo chick being found in a Meadow Pipit nest on one of the survey days, a reserve first!

On Coll at least cuckoos seem to be doing very well with birds apparently breeding in higher densities then is recorded at some sites in England, a good news story!

If you would like to visit Coll during Cuckoo season then plan a trip between late April and the end of June with birds at their peak in May.

Main image: Cuckoo perched on a tree stump. Ben Andrew