When I was writing Monday’s blog about cuckoos in Devon and on Dartmoor – see here I came across a reference about cuckoos I hadn’t seen before. I found out via the internet that it referred to a PhD carried out by Chloe Denerley from the University of Aberdeen (funded by RSPB and Natural England) on the conservation and ecology of cuckoos in NE Scotland and Devon. This blog is about that PhD and what it might mean for Dartmoor’s cuckoos – if you want to read here work you can download it here. It is a PhD thesis but it is surprisingly readable and the discussions are not too technical i.e. it is accessible to the interested lay person!
A cuckoo (taken in Northamptonshire in 2014 by my friend Steve Brayshaw)
It is a very valuable, important and well conducted piece of research and I commend it to all those interested in the conservation of cuckoos. The studies areas as mentioned above are the heathy areas of NE Scotland and the lowlands and uplands of Devon. Trying to summarise a 200+ page thesis in a few paragraphs isn’t easy but I will try using Chloe’s ‘key facts’. Here conclusions below are in italics, my commentary isn’t.
Cuckoos parasitise different bird species depending on the habitat – in semi-natural grasslands/heathland they lay an egg in meadow pipit nests, in farmland they parasitise dunnock nests and in wetlands they specialise on reed warblers.
Regarding cuckoo hosts (i.e. meadow pipit, dunnock and reed warbler) Chloe concludes when commenting on the declines of cuckoos across much of the countryside:-
Part one – cuckoo hosts
The probability of cuckoos being retained increases with meadow pipit abundance where semi-natural cover is high
Dunnocks are associated with farmland. No clear relationships between dunnocks and cuckoo presence was identified
Meadow pipits and cuckoos are both associated with semi-natural habitats, making it difficult to untangle relationships between them.
The probability of cuckoos being retained increases with dunnock abundance and reed warbler presence where semi-natural cover is low.
The dunnock gens may now be scarce
The abundance of hosts does not appear to be a strong driver of cuckoo declines
Cuckoos are not found in open areas and are associated with woodlands (need for vantage points?)
Cuckoos have declined severely in agricultural habitats
when meadow pipits are common the chance that cuckoos will breed successfully is higher (meadows pipits have declined in Devon since the 1977-85 survey compared to the 2007-13 work from 812 tetrads to 458 tetrads. Meadows pipits on Dartmoor have declined a bit but are still common.
Dunnock is the favoured host in farmland – dunnock has not significant declined in Devon between the 1977-85 survey and the 2007-13 one
In the ‘lowlands’ cuckoos do best where dunnock populations are high and reed warblers occur – that doesn’t occur in many places e.g. Slapton Ley?
Cuckoos that parasitise dunnocks, for example, are genetically programmed (because of egg mimicry) to only parasitise dunnocks, the same applies to meadow pipit cuckoos and reed warblers cuckoos – these species specific cuckoos are known as ‘gens’.
As the ‘dunnock dependant cuckoos’ have declined so much in Devon they may now be scarce and unable to re-populate the countryside if conditions improved.
The decline of the cuckoo does not appear to be associated with the abundance of meadow pipits (in semi-natural habitats), dunnock or reed warblers.
Where countryside is open and treeless cuckoos cannot view the terrain and find host species or prey.
Cuckoo populations have crashed in lowland Devon where agriculture is intensive i.e. most of it …… lowland Devon is now an intensive agricultural county.
The major conclusion of part 1 of Chloe’s work tells us that the presence/abundance of host species – meadow pipits (at least in semi-natural habitats) , dunnock and reed warbler is not responsible for the decline of the cuckoo. The loss of the meadow pipit in lowland Devon though, through agricultural intensification may well have played a part.
So …. if it is not the host species perhaps it is the food of the cuckoo – the so called ‘cuckoo prey’.
Part 2 Cuckoo prey (i.e. large hairy moth caterpillars)
Changes in moth abundance vary by habitat. The steepest declines have occurred in improved grassland and woodlands, while abundances in semi-natural habitats have increased slightly.
Cuckoo prey species of moth have undergone greater declines than other moths.
The probability of cuckoo presence increases with the abundance of moths known to be cuckoo prey.
In recent years, moth abundance was higher in semi-natural grassland and heath than in arable or improved grassland habitats
Decline of cuckoo prey in farmland may drive cuckoo declines
This work is extremely interesting – basically, intensively farmland throughout much of Devon no longer supports enough large hairy caterpillars and as a result there is not enough food for the cuckoos to eat – therefore they are now absent.
Butterfly Conservation published a report in 2013 – The State of Britain’s Larger Moths and this showed a number of species of moth had declined by over 75% between 1968 and 2007 – one of these was the Garden Tiger which had declined by 92%. The caterpillars of the garden tiger are known as ‘woolly bears’ and are thought to be one of the staples in the diet of England’s cuckoos.
Garden tiger – I can’t remember the last time I caught a garden tiger in my moth trap – maybe 15 years ago …
The woolly bear – I do see these more often – this one was taken on the Isles of Scilly
So what does all this mean for Dartmoor? It certainly isn’t time to be complacent – yes Dartmoor has a strong populations of cuckoos but as my blog yesterday showed cuckoos have declined by 10% over the last 25 years. My blog also demonstrated that some parts of the moor are better than others for cuckoos. The 10km square SX67 is particularly good – it includes Two Bridges, Postbridge and Dartmeet – as every tetrad (i.e. 25 for the 10km square) had records for cuckoo. That area is diverse – there are areas of semi natural grassland, woods, bogs, extensive areas of heath and scrub.
In the past the garden tiger would have been an important prey item for cuckoos on Dartmoor but I just don’t know how common or rare they now are on Dartmoor. In the garden tiger’s absence the ‘big four’ caterpillars on Dartmoor which cuckoos are likely to feed on are the fox moth, the emperor moth, the oak eggar and the drinker. I say ‘likely’ because nobody really knows what Dartmoor cuckoos eat but these four species are large hairy and poisonous caterpillar which cuckoos elsewhere are anecdotally known to favour.
Fox moth caterpillar
The first three species of the ‘big four’ feed on heather and as I have written before (see here for example) that there are places on the Moor where heather is now much less common than it was before.
This is the extent of heather communities on the National Trust’s Upper Estate in 1990
This is the extent of the heather in 2003
The message from Chloe Denerely’s work is clear – if there isn’t enough cuckoo prey (large hairy caterpillars) then cuckoos will disappear whether that is in the lowlands or the uplands. I very much suspect that the differences in cuckoo abundance on Dartmoor is driven by cuckoo prey abundance and that the condition of heather communities on the moor is inextricable linked to that.
We really need some research into this topic and I have a feeling that is exactly what a researcher at the University of Exeter is now doing.