Understanding the decline of cuckoos

Blog post by Prof. Jeremy Wilson, Head of Research in Scotland, RSPB Centre for Conservation Science

England’s loss but Scotland’s gain

Once part of the soundtrack of an English summer, the call of the cuckoo is now a fading memory for many. We have lost over three-quarters of the UK cuckoo population since the 1980s yet know little of the reasons why.

Yet more intriguingly, changes in cuckoo numbers vary dramatically across the UK; in England, the Breeding Bird Survey organised by the British Trust for Ornithology (BTO) has documented a 70% decline since 1995, whilst the Scottish population has increased by 30%, a trend apparent especially in the Highlands (Figure 1).

More locally, it is clear that cuckoos are faring much better in areas of heathland, especially in the uplands, than in the farmed lowlands. Nowhere is this more apparent than in Devon, where cuckoos were widespread across the county in the 1980s, but are now confined to the margins of Dartmoor and Exmoor.

Fig. 1. Changes in abundance of Cuckoos between the 1988-91 and 2007-11 Atlases.

A unique challenge

Trying to understand these complicated patterns of change presents a unique challenge. Adult cuckoos are present in the UK for only a short time each year, arriving from April and often gone by July.

And of course they are brood parasites so that the habitat and feeding conditions necessary for cuckoo breeding success must match those of their main hosts – meadow pipits, reed warblers or dunnocks – whose usurped nesting attempts raise cuckoo nestlings to fledging.

On top of all that, for the brief period that cuckoos are with us, they specialise in feeding on large, hairy moth caterpillars such as the ‘woolly bears’ of the garden tiger (Fig. 2), that are toxic to most other birds.

Fig. 2. A ‘woolly bear’ - the caterpillar of the garden tiger moth and a key prey item for adult cuckoos. Photo credit: © Jeremy Wilson.

Exploring the patterns

The stark patterns in the fortunes of Cuckoos became the focus of a PhD study undertaken by Chloe Denerley and supported by the University of Aberdeen, the RSPB and Natural England.

Chloe worked at two scales – both in the field in a study area in Devon, but also using national data sets describing populations of cckoos and their hosts (the BTO/JNCC/RSPB Breeding Bird Survey), habitats (the Centre for Ecology & Hydrology’s Land Cover Maps) and population changes of moths (Rothamsted Research’s light trap network).

At both scales, Chloe analysed the breeding distribution of cuckoos in relation to habitat variation, the abundance of hosts and the abundance of moths whose caterpillars are a key food of adult cuckoos.

A female Cuckoo on Dartmoor. Photo credit: © Professor Charles Tyler, University of Exeter

In Devon, cuckoos were found more often in areas with more semi-natural habitat and more meadow pipits (but fewer dunnocks) and where, later in the summer, higher numbers of moths were light-trapped whose larvae are cuckoo prey.

Nationally, cuckoos have become more associated with upland heath with meadow pipits, and with wetland habitats with reed warblers, and the distribution of cuckoos has shifted from south to north within the UK.

The abundance of moth species preyed upon by cuckoos has declined four times faster than that of other moths. The abundance of these moths has shown the sharpest declines in grassland, arable and woodland habitats and has increased in semi-natural habitats (heaths and rough grassland).

The conservation implications

Overall, Chloe’s findings suggest that agricultural change in the UK may well have played a part in driving cuckoo declines, pushing the birds increasingly out of the farmed countryside and into heathlands and the uplands.

The ideal next step to test the feasibility of restoring Cuckoo populations more widely would be a replicated, landscape-scale intervention.

This should focus on the coordinated restoration of species-rich grassland, reduced pesticide use on arable land, hedgerows with less-than-annual trimming and minimal understorey disturbance, and grass field margins not subject to agrochemical application, ideally located close to a remaining centre of cuckoo population such as Dartmoor.

Equally, Cuckoos might be expected to respond well to existing large-scale ecological restoration projects across a wide variety of landscapes from wetlands, to rewilding interventions in agricultural landscapes, to the uplands. Coordinated monitoring of moth, host and Cuckoo numbers across such projects could tell us much about the long-term prospects for reversing Cuckoo population losses in the UK.

Read the paper at https://onlinelibrary.wiley.com/doi/full/10.1111/ibi.12612

Reference

Denerley, C., Redpath, S.M., van der Wal, R., Newson, S.E., Chapman, J.W. & Wilson, J.D. (2018) Breeding ground correlates of the distribution and decline of the Common Cuckoo Cuculus canorus at two spatial scales.