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.
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.
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.
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.
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.