Guest blog by Dr Lowell Mills – recent PhD graduate from the University of Exeter, in partnership with the RSPB
In a University of Exeter and RSPB study published this month, we improve understanding, using DNA-sequencing and photographs taken by the public, of the range of caterpillars selected as prey by cuckoos, but also find some unexpected other prey items in the diet. Better understanding the diet of cuckoos is crucial to inform conservation management during their short stay on the breeding grounds in the UK.
Cuckoos have been declining in the UK and many other parts of Europe, but determining the underlying causes is challenging. They are long-distance migrants between Africa and Eurasia, but are also ‘brood parasites’ that depend on small songbirds to raise the young cuckoo. The species is also often secretive and can hold a ‘home range’ of many square miles. In summary there’s a lot to unpack for conservationists trying to work out which factors are driving decline.
One clue to answering this is that cuckoos show varying population trends between regions and habitats. Breeding numbers have declined more in farmland and woodland than on open grasslands and moorland in the UK, and have declined in England while increasing in Scotland. This suggests that declines may be linked to changes in factors local to the breeding population, such as breeding habitat or food supply.
Cuckoos have declined in Britain overall but show different patterns of change between regions and habitats. Could food supply be driving this?
Cuckoo diet has not been studied in depth since declines began, and has relied on field observation that might overlook small prey. As part of a PhD with University of Exeter in partnership with the RSPB, we aimed to better identify prey of the cuckoo in refuge upland habitat using recent DNA-based methods, and update the national picture of cuckoo diet by crowd-sourcing photographs of them handling prey, a technique also used recently by the RSPB’s Project Puffin.
Design of the study
While the south-west of England has seen a decline in cuckoos, the grasslands and moorland of Dartmoor National Park represents a refuge for the species. The goal for testing cuckoo diet in uplands like this was to 1) collect as many samples of their droppings as possible, 2) sequence (read) DNA to identify the invertebrates that had been eaten, and 3) note what species were most commonly detected.
Typical view of the mixed semi-natural upland landscape of the study area at Holne Moor, Devon © Lowell Mills
Collecting and analysing cuckoo droppings
Some droppings were collected while ringing cuckoos under British Trust for Ornithology (BTO) license, but we also developed a new technique to ‘pick up after’ cuckoos to help sample from more birds and sites. We were able to watch cuckoos around the sites and collect fresh samples from beneath trees, rocks and walls where they perched.
We confirmed in the lab whether a sample came from a cuckoo, then pulling out the invertebrate DNA and comparing it to an online database of known sequences to identify the species.
Use of crowd-sourced photographs
High resolution images of cuckoos uploaded to online platforms included many showing readily identifiable prey. Study co-author Barry Henwood has expertise particularly in identification of caterpillars. Prey of fully-grown cuckoos was identified from across Britain.
What are Cuckoos eating? © Charles Tyler
‘Hairy’ caterpillars have long been suggested to be the staple diet of cuckoos, and indeed they were the most frequent prey in faecal samples, especially drinker moth. However, through the ability of DNA studies to detect prey of a range of sizes and in trace amounts, we identified grasshoppers and large flies like snipe-flies and crane-flies to also be frequent food sources, plus occasionally beetles, bugs and even stoneflies.
Similarly, while drinker moth caterpillars were identified as most frequent from the crowd-sourced images nationally, cuckoo adults were found to occasionally feed on earthworms, while juveniles fed on a different range of caterpillars, mainly of cinnabar moth and sometimes adult moths.
A large caterpillar of the drinker moth, found within study faecal samples; and in crowd-sourced photographs of cuckoos feeding throughout Britain © Lowell Mills
What does this mean for cuckoo declines?
Key take-home messages from this study are that cuckoos are fairly generalist predators of invertebrates, but key prey appear to be species vulnerable to intensive land management. Drinker moths may be affected by drainage and re-sowing of grasslands reducing their foodplants, while other moth species detected may take heavy losses of eggs or larvae under hedgerow flailing practices. Grasshoppers lay eggs into undisturbed soil and could have high adult mortality under silage cutting practices.
The good news is that areas of experimental low-intensity land management already exist in Britain and Europe, for scientists to monitor invertebrate and cuckoo populations, and determine whether such approaches to land and agriculture support prey populations sufficiently to halt or reverse declines.
For the full paper in Journal of Avian Biology: Using molecular and crowd‐sourcing methods to assess breeding ground diet of a migratory brood parasite of conservation concern
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Fascinating study. Reinforces how important Ragwort is, not only for the massive suite of inverts that it supports but now we know it's important for Cuckoos as well.