An imaginary cuckooland

Today, had I not been otherwise engaged, I would have gone to Dartmoor to search for cuckoos. I might have gone to Emsworthy Mire or to Gidleigh Common by Scorhill Farm or even to Throwleigh Common near to Great Ensworthy. Last year I listened to and saw cuckoos at these places, but sadly this year I will have to imagine them but I know they will be there!

To get my mind running I am assisted by some poetry – no less a person than William Wordsworth, describing his joy at hearing a cuckoo return to the Lake District.

O blithe New-comer! I have heard,
I hear thee and rejoice.
O Cuckoo! shall I call thee Bird,
Or but a wandering Voice?

and

O blessed Bird!
the earth we pace
Again appears to be
An unsubstantial, faery place;
That is fit home for Thee!

And John Clare, a contemporary of Wordsworth writing about cuckoos in his Northamptonshire village of Helpston.

The cuckoo, like a hawk in flight,
With narrow pointed wings
Whews o’er our heads – soon out of sight
And as she flies she sings:
And darting down the hedgerow side
She scares the little bird
Who leaves the nest it cannot hide
While plaintive notes are heard.

The cuckoo however, is a marmite bird, despite the millions of years of its evolutionary journey where it has re-invented itself as an exploiter of other birds, its habits repulse others. A paradox bird – the herald of spring and new life on one hand and the cruel deceiver taking life on the other.

Ted Hughes captures the latter view.

The cuckoo’s the crookedest, wickedest bird,
His song has two notes but only one word.

He says to the linnet:  Your eggs look so ill!
Now I am the doctor, and here is my pill.”

Within that pill, the cuckoo-child
Crouches hidden, wicked and wild.

He bursts his shell, and with weightlifter’s legs
He flings from the nest the linnet’s eggs.

Then bawls to the linnet:  “Look at me, Mam!
How quickly I’ve grown, and how hungry I am!”

She thinks he is hers, she is silly with joy.
She wears herself bare for the horrible boy.

Till one day he burps with a pitiless laugh,
“I’ve had enough of this awful caf.

And away he whirls, to Cuckooland,
And leaves her to weep with a worm in her hand.

What is so curious about this poem is the species of bird that Hughes has chosen to become the victim  of the cuckoo – a linnet. Very occasionally cuckoos will lay an egg in a linnet’s nest but it is a fatal mistake for the cuckoo as well as the linnet. Linnets are seed eating birds whilst cuckoos need a diet of insects; the cuckoo chick hatches, ejects the linnet’s clutch of eggs and then starves to death. Hughes was a good naturalist and a keen observer of nature, maybe I have missed something, maybe it’s just creative licence.

Part of the inherited evolutionary genius of the cuckoo is its ability to mimic the egg patterns of its host in both colour and markings. What makes this all the more extraordinary is that cuckoos parasitise a range of host birds. Wordsworth’s cuckoos’ would lay eggs in meadow pipit nests whilst Clare’s would have chosen the dunnock. In areas where there are extensive wetlands cuckoos utilise the nests of reed warblers. These three species of passerine birds have very different coloured and patterned eggs and as a result cuckoos have evolved into host-specific races.

My own interest in cuckoos developed when I worked at Wicken Fen in Cambridgeshire and met the legendary Professor Nick Davies who used the reserve as his laboratory to study cuckoos and their interactions with reed warblers. Nick is a brilliant academic, a lovely, lovely man and very generous with his time. Ecologists will know of him as well through his textbook ‘An Introduction to Behavioural Ecology’.

In his book ‘Cuckoos – cheating by nature’ he wrote

Without a moment’s hesitation, the warbler bows deep into the enormous mouth to deliver the food ….

I am amazed by what I have just seen …

Why, when confronted by a young cuckoo, so different in appearance and far too big to be one of their own chicks, are the warblers apparently so stupid?

I am amazed by the cuckoo chick too. How does it stimulate the little warblers to bring enough food?

And why do adult cuckoos abandon their young and entrust them to another species?

In his book Nick goes on to answer these questions, based on his own research at Wicken Fen along with his numerous PhD students and other academics around the world. It is the most amazing story – if you love nature, are puzzled by evolution and curious about animal behaviour you will love this very readable book – it will make lockdown time fly by.

And now …. I have been to ‘Cuckooland’ without leaving my house.

 

Heather beetle damage on Dartmoor 2019

I have been very surprised how much Heather Beetle damage there is on Dartmoor this year. The Heather Beetle larvae hatch in June and then feed on the young leaves and shoots. As a result the affected parts of the heather plant turn orange brown.

Heather Beetle damage. It is very characteristic and eye catching.

Last week I walked from Rowtor on Okehampton Common up the military road to Observation Post 15 and then down to Ockerton Court. All the way along the track there are signs of extensive damage to the heather plants – I would estimate that over 90% of plants are affected.

Work carried out in the north of England and in Scotland suggest that the larvae are active until the end of August when they drop down into the litter and pupate into adult beetles. Despite three separate searches on Okehampton Common, the Forest of Dartmoor and Headland Warren Common I only managed to find three larvae and one adult beetle.

Here is a Heather Beetle larvae on Okehampton Common eating the few remaining green leaves of the plant.

I suspect that the absence of larvae and adults during my searches in the first week of August means that the larvae have already dropped into the litter and are beginning to pupate – as a result larvae and adults are not visible. However the very extensive areas of damaged heather indicates that they have been very active in June and July.

It is possible for the heather to recover from this attack and I will be monitoring it to see if it does. However parts or all of the heather plant can be killed. When this happens the shoots turn from orange brown to grey.

This is mature heather at Ockerton Court which has been killed by Heather Beetle

In this image the areas of dead heather (darker brown bits) are being over run by Purple Moor Grass (Molinia caerulea) – the bright green shoots of this year’s growth and the light brown leaves from last year. Molinia is unable to replace heather whilst the heather is alive but can and does do so when it is dead.

Heather Beetle is a seriously under-recorded species on Dartmoor – the National Biodiversity Network database has just one record and the is from Fingle Woods and not from the high moor!

There is clearly a need to gather more records ….

There is anecdotal evidence nationally that Heather Beetle attacks are getting worse and it has been suggested (based on research from the Netherlands) that the severity and frequency of Heather Beetle outbreaks is linked to the levels of atmospheric pollution – particularly nitrogen levels.

Dartmoor receives high levels of nitrogen deposition as a result of its high rainfall – Natural England have reported that Dartmoor receives 24kg / ha / annum of nitrogen (as NOx) which is damaging the blanket bog and mires. This high level of nitrogen deposition may also be responsible for the high levels of Heather Beetle damage.

Heather Beetles are a natural part of the moorland wildlife community and historically damage to heather was limited except in the ‘outbreak’ years. Last year when I was walking the Commons in July I also noticed extensive areas of affected heather – at this point in time heavy attacks appear to be frequent – maybe even annual.

There are implications for wildlife, conservation and hill-farming as a result of these serious Heather Beetle attacks.

  1. The heather plants themselves are either killed or remain stunted
  2. The species of wildlife which feed on heather shoots are also impacted – this includes moth species such as the Emperor Moth and the Fox Moth, whose hairy caterpillars are important prey items for one of Dartmoor’s iconic and successful birds – the cuckoo.
  3. Heather has long been a conservation indicator for the condition of Dartmoor’s Commons and historically grazing levels were reduced significantly to reduce overgrazing pressures to conserve heather.
  4. Heather is also a winter food for sheep on the Commons, if the amount of heather generally is significantly reduced as a result of Heather Beetle attacks it put pressure of the remaining plants that have survived.

Ironically it is thought that the severity and frequency of Heather Beetle attacks has increased because the nitrogen has made the young shoots and leaves more nutritious, it is also reported that sheep preferentially graze the new shoots too for the same reason.

There is a dearth of information on Heather Beetle issues on Dartmoor but from my own observations this year and last it is a potentially serious and widespread problem. However it would also appear that the problem is not universal across the moor. I searched for it in the heather stands around the Warren House Inn  and found Heather Beetle attacks to be minor – perhaps this level of damage is the natural level – whereas the levels seen on Okehampton Common, the north part of the Forest and on Headland Warren Common are the outbreak levels.

It seems to me that there is a clear need to better understand the Heather Beetle situation on Dartmoor, this would be in the interests of Natural England, the Dartmoor National Park Authority and the Dartmoor Commoners Council. The time to survey for the impacts of Heather Beetle is July and August. Maybe a bit of ‘Citizen Science’ could come to the rescue?

I would also be interested to hear from people who have found the characteristic orange brown stands of heather this year on Dartmoor.

I’ve written before about Heather Beetles and this link takes you to my blog which contains further information and some references you can download.

A day on St Agnes

Spent the day on St Agnes yesterday

On the tripper boat …

Gugh Bar joins At Agnes with Gugh – the most famous tombolo in the world!

Pleased to see a small colony of kittiwakes surviving on Gugh – Britain’s most endangered seabird as a result of climate change and rising ocean temperatures.

Looking over to Gugh

The Devil’s Punchbowl on Wingletang Down

Cuckoo on Wingletang Down

The Nag’s Head

Male wheatear –  a passage migrant on Scilly

A colony of fulmar

An unnamed granite outcrop on Castella Down

Male Stonechat

The Troytown maze

A rock pipit

Pretty much my favourite island

Tigers on the wing

Two of my favourite moths are currently on the wing.

Jersey tiger
This is a Jersey Tiger that was in my trap last night. According to Waring, Townsend and Lewington in their Field Guide to the Moths of Great Britain this is a National Notable B species – meaning it occurs in less than 100 10 kilometre squares. It is however common along the south coast of Devon and it has now spread up as far as Sussex. In some places such as Exeter it occurs inland.

Jersey tiger 2
As can be seen in this photograph is has bright red/orange underwings.

Garden tiger 2
This is a Garden Tiger – the one I photographed on Bryher last week in the Isles of Scilly

Garden Tiger
Again the species has bright red/orange underwings which act as a would be deterrent to potential predators. Butterfly Conservation has carried out some research on the status of the Garden Tiger – they found that between 1968 and 2007 the species had declined by 92% – a species in real trouble.

Garden tiger caterpillar 1
This is the caterpillar of the Garden Tiger – often referred to as ‘wooly bears’. These are one of the favourite prey items of the cuckoo which is another species in major decline. Work being carried out at Exeter University might show that the two are connected and that the intensification of agriculture in recent years has played a negative role.

I’d be interested to know if anyone has seen a Garden Tiger so far this year?

New BTO cuckoo research and what it might mean for Dartmoor

The British Trust for Ornithology (BTO) has just published a paper in Nature Communications entitled “Population decline is linked to migration route in the Common Cuckoo”. Scientists at the BTO led by Chris Hewson have been using tiny satellite trackers attached to cuckoos so that they can plot their progress and routes. This is what part of the abstract of the paper stated

” ….by tracking 42 male Common Cuckoos from the rapidly declining UK population during 56 autumn migrations in 2011–14. Uniquely, the birds use two distinct routes to reach the same wintering grounds, allowing assessment of survival during migration independently of origin and destination. Mortality up to completion of the Sahara crossing (the major ecological barrier encountered in both routes) is higher for birds using the shorter route. The proportion of birds using this route strongly correlates with population decline across nine local breeding populations.”

You can download the paper here along with the supplementary tables here. Nature Communications have kindly made this paper ‘open access’ for there is no charge.

BTO cuckoo migration map

This is a graphic from the recent BTO News magazine which summaries the findings regarding route choices. In essence UK cuckoos either use the eastern route through Italy / The Balkans or the western route through France / Spain when migrating south. Interestingly all birds use the western route to return to Britain. The research statistically proved that mortality of birds travelling south via the western route was significantly higher than for birds using the eastern route even though the western route is shorter.

The paper includes this graphic with the following caption ‘Cuckoo migration route use and breeding population change for each tagging location’.

Screen Shot 2016-07-26 at 12.30.31

From this we can see that 6 Dartmoor birds were tagged four used the eastern route and two used the western one (yellow/red pie charts). The undying pink / grey dots on the UK map show the change in abundance of cuckoo populations across the UK as measured by the BTO Atlas Projects in 1988–91 and 2007–11. Dark pink indicates a good increase and dark grey a big decrease (see the colour scale on the right side of the map above +0.75 = 75% increase in abundance and -0.75 = 75% decrease in abundance between the two survey periods. It is currently not clear why the western route is more hazardous – it might be land use change and drought in southern Spain – more research on this required.

The BTO team then correlated the route data with the abundance change data – this is shown in Supplementary Table 2 – I have turned that data into a little graph so it is easier to understand.

Screen Shot 2016-07-26 at 14.00.47

The axis which runs along the bottom of the graph shows the proportion of cuckoos using each route, 1 = all birds using the eastern route and 0 = all birds using the western route. The Dartmoor birds score 0.58 = 58% of Dartmoor cuckoos use the eastern route and 42% use the western route. The scale which runs up the graph shows the change in abundance of cuckoos between the two BTO Atlas survey projects. The Syke / Kintail population score 0.433 = a 43.3% increase. The Sherwood Forest population score – 0.465 = a decrease in abundance of 46.5%. The Dartmoor population is -0.242 = a decrease in abundance of 24.2%. The BTO work proves that there is a statistically significant correction between these two factors i.e. the proportion of birds using the western route strongly correlates with population decline across nine local breeding populations.

With the publication of the Devon Bird Atlas late last year we already knew that cuckoos in Devon were in big trouble.

Cuckoo3
This is the distribution of cuckoo in Devon between 1977-85

Cuckoo2This is the distribution between 2007 and 2013 – approaching a 75% decline in 20 years

The Devon Atlas does show a strong population still on Dartmoor but as I have argued before this population is also declining (see here). I produced a graphic which showed the changes on Dartmoor between the two survey periods.

Cuckoo change data
Four 10 x 10km squares had increased numbers of tetrads (2x2km squares) where cuckoo bred, three showed no change but nine showed declines. Overall cuckoo bred in sixteen fewer tetrads  in 2007-13 compared to 1977 to 85.

The data in the BTO paper ascribes the decline to 24.2% on Dartmoor using the BTO Atlas data (comparing 1968-72 with 2008-11). The paper also clearly shows that the 42% of Dartmoor cuckoos which migrate via the western route suffer a higher mortality rate than the 58% which use the eastern route which goes some way to explaining the decline.

However mortality rate on migration cannot be the whole story – it cannot explain the 75% decline in Devon. Away from Dartmoor and Exmoor the cuckoo is now virtually extinct in the county when 30 years ago it was common. The change in land use in lowland Devon is almost certainly the culprit here. I have written about this before – see here and have discussed the decline in the population of large hairy caterpillars which cuckoos are so fond of.

Cuckoo 3
A Dartmoor cuckoo this year at Emsworthy

Fortunately further  research is being conducted at Exeter University by Professor Charles Tyler and his team which includes Sara Zonneveld and Lowell Mills who are both conducting PhD research on this very topic. I am very much looking forward to hearing about what they have discovered. Their work along with the BTO’s migration work (and other relevant research from the RSPB) will I hope help us piece together what has happened to our cuckoos so that we might have a chance in the future to do something about it.

I was also pleased to see Devon Birds credited in the acknowledgements section of the BTO paper. I understand that Devon Birds provided funding to help acquire some of the satellite trackers.

Cuckoos in Plymouth with Nick Davies

Last night I attended a lecture, at Plymouth University organised by their Biological Sciences Department and co-funded by the Linnaean Society, given by Professor Nick Davies of Cambridge University entitled Cuckoo: Cheating by Nature.

Nick Davies 2
The lecture summarised Nick’s lifetime work on the behavioural ecology of the cuckoo – most of his work having been carried out at Wicken Fen in Cambridgeshire.

Nick Davies
The lecture was based around his recently published book – Cuckoo: cheating by nature which I reviewed last year see here. In my opinion it is the best natural history book I have ever read.

Nick Davies 4Cuckoo: cheating by nature has also recently won the British Birds / British Trust for Ornithology Book of the Year 2015 – here is a photo of Nick receiving his award from Dawn Balmer of the BTO (from BB Magazine March 2016)

 

 

It was a fabulous and well attended lecture. It was really good to see and briefly talk to Nick again –  I left Wicken Fen 12 years ago where I was the Property Manager for the National Trust. Happy days!

During his talk Nick made a couple of references to the cuckoo work being carried out on Dartmoor by Professor Charles Tyler and his team. That team – the Dartmoor Upland Bird Nest Group have been fundraising to help with this season’s field work – to date they have raised over £3000 – you can read their latest update here where you can also donate to help the research on cuckoos.

This research is essential – Nick ended his talk by describing the decline of the cuckoo at Wicken Fen -down from 15-20 ‘pairs’ a couple of decades ago to only 4-5 ‘pairs’ now. Dartmoor and Exmoor remain as strongholds for the species now in England and it is vital that the ongoing research unravels the reasons behind these wider declines so that these can be addressed.

As Nick said at the beginning of his talk – only six weeks to go and the cuckoos will be back – can’t wait.

 

A trip to Holne Moor – cuckoos, scrub and flood prevention

I spent the afternoon yesterday with Kevin Cox, who lives in the Mardle Valley, is an RSPB Council member and heavily involved with Devon Birds. We talked about Devon birds, Devon Birds and the management of Dartmoor’s commons. Kevin has recently purchased part of Holne Moor from South West Water.

Holne 1
Holne Moor overlooking Venford Reservoir.

We went up to Holne Moor to have a look around. A very interesting visit for me. This is the key bird research area I have written about recently – the place where Exeter University’s Professor Charles Tyler, his team of research students and nest finders have been working (The Dartmoor Upland Bird Nest Group) – see here and here. This is the area where some of the key cuckoo research is taking place as well as being an area which supports high population densities of whinchat and meadow pipit.

Holne 4
The moor is grazed and has a swaling programme but does have quite a lot of small trees dotted around the landscape – cuckoos need these small trees so that they can survey the landscape and see where the meadow pipit nests are. On many commons now these dotted isolated trees are absent and new regeneration is now difficult due to the grazing and burning pressure.

The area is also very interesting as it gives a clue as to how natural flood management measures might work on Dartmoor in the future and play a part in ‘slowing in the flow’. South West Water have retained a belt of land around their reservoir at Venford. This area has been fenced off.

Holne 2

In this photograph you can clearly see the fence line – with grazed moorland to the right and the lightly grazed enclosure to the left. You can see that patches of light scrub have developed in the closure.

Holne 3
Here is another view of that enclosure.

These two photographs tell me a couple of things.

Firstly, if Dartmoor was not grazed, scrub and eventually woodland would quickly develop – the George Monbiot re-wilding scenario. Dartmoor is of course as I have said many times before an important historical and cultural landscape and therefore if the re-wilding scenario were to happen across the Dartmoor landscape then most of that would be lost. The landscape of Holne Moor is a good example of this as it has been ‘designated’ as a Premier Archaeological Landscape – see here for further details.

Atlas of Antiquities 1Jeremy Butler in his 5 volume Dartmoor Atlas of Antiquities sets out a detailed catalogue of the archaeological interest.

Atlas of Antiquities 2
The map and accompanying text details the importance of the area from the Bronze Age, through the Mediaeval period to the present.

The challenge for all those involved with the management of such places therefore is getting the balance right between archaeological interests and biodiversity – both of which are of European Importance. I have written about this challenge before and it seems to prove intractably difficult to solve even though all parties are in fact pretty much in the same place – i.e. everyone wants a grazed landscape.

As Kevin Cox said to me on site yesterday (I paraphrase) – the archaeology has survived on here on Holne Moor for thousands of years through the ebb and flow of vegetation and farming cycles, however at the moment there is a biodiversity crisis and we may only have 30 years to save some species such as the cuckoo. Surely there is enough flexibility and goodwill within the system to tweak a few management techniques and thereby work out how to enable the cuckoo (and whinchats, meadow pipits etc) to flourish (e.g. ensure there are perching places and enough food for cuckoos) – the work that the Dartmoor Upland Bird Nest Group are currently researching.

The second thing that the two photos above tell me is how quick and easy theoretically it will be to naturally add regenerating trees and scrub to the landscape in very small but strategic places so that natural flood management schemes can help slow the flow. If enclosures were erected around specific stream valleys the developing scrub would quickly emerge and add ‘hydraulic roughness’. The areas of grazing land lost would be tiny and as long as the Commoners were compensated and not penalised as the current ‘ineligible feature’ nonsense currently would do then surely this too is a win-win for everyone.

I thought yesterday was going to be dominated by Storm Imogen – it certainly seems to have around our coasts but inland it was pretty windy but in my experience was mostly dry and allowed me instead to make a new friend, see a new place and think more about Dartmoor and its management.Holne 5

 

New cuckoo research for Dartmoor

Following on my my two recent posts on cuckoos on Dartmoor (here and here) I have been in contact with Professor Charles Tyler and Dr Malcolm Burgess who are both co-ordinating and working on cuckoos on Dartmoor. There are two new PhD students working on cuckoos, one developing molecular methods that can be applied to definitively identify cuckoo prey species from their droppings and the other studying ground nesting birds including the cuckoo.

The second student is currently crowd funding so that the 2016 field season can be supported. Here the video setting out the aims of the project.

 

And here is the link you can follow to support the project – go for it!

More on Devon’s cuckoos and how to make Dartmoor better for them

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

So ….

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
Garden tiger – I can’t remember the last time I caught a garden tiger in my moth trap – maybe 15 years ago …

Garden tiger caterpillar 1
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
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.

Heather in cats 1&2 1990
This is the extent of heather communities on the National Trust’s Upper Estate in 1990

Heather in cats 1 & 2 2006This 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.

 

 

Cuckoos in Devon and on Dartmoor – a depressing story

On Saturday I wrote about the newly published Devon Bird Atlas and commented briefly on the decline of the cuckoo – see here. During the 1977-85 survey cuckoos were recorded in 1447 tetrads (a 2km x 2 km square), that is 78.9% of all possible tetrads in Devon.

Cuckoo3
This is the distribution map for cuckoo from Sitter’s 1977-85 Atlas – it pretty widespread.

The data from the 2007-13 Atlas is as follows: – the cuckoo was recorded in only 372 tetrads which is only 20% of all possible tetrads in Devon. That is a 289% decline compared to the Sitter’s data.

Cuckoo2
Here is the map from the 2007-13 Atlas clearly showing that massive reduction in distribution.

It is also clear from this map that the last stronghold for cuckoo in Devon appears to be on Dartmoor. I was keen to try and use the data from the two Atlases to see if the distribution on Dartmoor was stable (as suggested by the Dartmoor National Park Authority – see here) or was also contracting.

This was not an entirely straightforward proposition as it involved replicating each tetrad record in both the Atlases onto an Ordnance Survey Base Map and only including those records within the National Park. Around the edge of the National Park tetrads often straddled the boundary – to get around this I used a consistent approach between the two surveys i.e. both in.

Cuckoo1
This is the result for the 10 km square SX67 – red dots are the 2007-13 data and yellow dots the 1977-85 data. The numbers on the dots refer to the breeding status (i.e. 1- present, 2 – possible, 3 – probably, 4- confirmed).

Once I had crunched all the figures it showed that during the 1977-85 survey cuckoos were recorded in 202 tetrads and during the 2007-13 survey cuckoos were recorded in 184 – a decline of 9.8%.

The following maps show the tetrad data at a 10km square level.

Cuckoos 77-85Four 10km square with over 21 tetrad recorded in 1977-85

Cuckoos 07-13Only one during the 2007-13 data

Cuckoo change data
The cuckoo population seems to be strong in the middle of the moor but is contracting particularly in the north eastern corner.

The breeding status data is less easy to interpret but it does suggest that whilst more ‘confirmed’ breeding tetrad sites were found in the 2007-13 survey there were more ‘probable’ records in the 1977-85.

It needs to be emphasised that this data only tells us about distribution – it doesn’t help us understand abundance. The 2007-13 Atlas does include abundance figures but at this point we have nothing to compare it to.

A 10% decline in distribution in around 25 years in a national park is rather worrying. The north east corner of the National Park is probably the most intensively farmed which may explain some of the changes, but so might the decline of the meadow pipit and the decline of large caterpillars (the cuckoo’s preferred food) some of which feed on heather (which is also in many places in decline).

It is perhaps worth finishing by saying that although the Dartmoor data is worrying  the National Park remains the stronghold for the cuckoo in southern Britain. There is a lot of work going on at the moment trying to further understand the ecology and conservation of the cuckoo on Dartmoor and let us hope that one we know more we can then implement the forthcoming action plan and halt the declines.

If you haven’t already done so I strongly recommend you acquire a copy of the new Devon Atlas  – copies of the 1977-85 Atlas are also still available.