The Warbler Factory

Posted by Graeme Lyons , Thursday 25 April 2019 08:07

Butcherlands. The most exciting, unpredictable and consantly changing Common Bird Census I have ever done. This is the eighth year I have carried out the four visit CBC there and it's amazing how much it's changed. Eighteen years ago this was all arable on Wealden Clay and was fenced and left to be restored to something more natural with pulse grazing by cattle currently pretty much the only management. No planting occurred and there are at least 30,000 oaks out there alone. Before looking at some of the detail though I am gonna cut right to the chase and tell you about what happened yesterday morning. First though a recap from my first visit on the 3rd April 2019. I had a Hawfinch fly over (my first since the previous winter's influx) and then surprisingly a male Dartford Warbler called repeatedly from a bramble bush. Not all that unusual to pick them up in bramble scrub away from the heaths outside of the breeding season (I've seen them in Sheepcote Valley on the edge of Brighton for example). 

Yesterday morning, the warblers were in full song. Whitethroats were EVERYWHERE. I recorded 44 singing males across the site. So when I got to the exact same area and heard Dartford Warbler calling again, I was pretty shocked. "It's holding territory!". I bungled the video last time so I got a quick video of it calling and you can see it fly from the bush to the left.

I thought I really should try and hear it singing, so I headed a few bushes away and crouched down out of view. It didn't work and the bird came towards me, scolding me from just a few metres away. I grabbed another video as it did so and it started singing a tiny bit. However what happened next was incredible. A female popped up out of the same bush! "THEY'RE HOLDING TERRITORY!". Some context here, there isn't any heather at Butcherlands, there is however hectares of low bramble scrub. This part of the site is on the edge of a sea of undulating bramble. It's also south west facing with a slightly sandy soil (more Common Bent/Cat's-ear than Heather). So it's hot, dry and with a structural component close to heather/low gorse. I never thought that this would be a thing at Butcherlands but these birds are much less fussy abroad. It may well be that this structural type and temperature envelope is rarely provided outside of the heaths in the UK.

Has anyone else encountered this in the UK before?

Yesterday morning my brain was totally fried from all the warblers. After this encounter I was doubting every Whitethroat that called, after all, if one pair of Darties could nest there, why not more? I really doubt I would have picked them up with so many Whitethroats there if I hadn't found the male three weeks ago. So many questions. So exciting!

I actually recorded over 75 singing warblers yesterday! My rule for difficult species pairs is this: if you're not sure, it's the commoner one. Think I'll call that 'Lyons' Razor'. It certainly came in handy with a few Blackcaps today. A Blackcap AND a Garden Warbler taking turns to sing from the same willow nearly made me lose my mind though.

Whitethroat - 44
Chiffchaff - 11
Garden Warbler - 8
Blackckap - 8
Lesser Whitethroat - 4
Dartford Warbler - 1

That's 76 warblers in all. Plus ten Nightingales and a Cuckoo. 

How has this changed over the last eight years though? With provisional data from yesterday's visit (this will change) the warblers look like this. A highly significant increase from linear regression. Only Chiffchaff and Blackcap came out as stable (as they are probably at capacity in the hedgerows and fields edges - for now).

And then like this if you break them down to species.

It's not all winners though. Nightingales have dipped a bit (although this trend is not significant) and Skylarks have nearly disappeared completely (a significant decline), they are still holding on this year though.

But look at the overall number of territories for all species (no suggested data here for 2019) shows a significant increase.
If you break the data down to Red & Amber listed Birds of Conservation Concern and compared to the Green listed, there is no significant trend with the scarcer birds but a highly significant one with the green birds. Showing that the generalists are doing better/quicker than the specialists. You would expect this though. This graph would look very different if Whitethroat was still amber listed but that is not something to wish for!

Species-richness of birds holding territory looks like this from 2012 to 2018 and shows no trend being about as level as it could be.

Linnet and Dunnock are two of the other species that have colonised the centre of the fields and have rocketed up significantly over the period. However the star of the show is yet to arrive. Last year FOUR Turtle Dove territories were noted and this chart shows how they have fared over the period. Wow, wasn't expecting to analyse the whole data set. Got a bit carried away there with linear regression.

I think the success of a project like this is when the site can sustain all of the species that the site has the potential to carry. So losing the red-listed Skylarks to the red-listed Linnets completely doesn't seem right to me just because we don't have an easy way of controlling the scrub. It's always early successional habitats that lose out without a constant effort to keep them open, that's why the associated species are rare. But I am also a fan of not trying to have everything everywhere all of the time. Additionally, scale is a factor here and it may not be possible to have all the successional states in 80 ha in a functional way. It shouldn't stop us trying though. How far back do you go though? After all, soon after the project started some 18 years ago (way before monitoring began) there were breeding Lapwing out there. The site would have to look very different to support them again, not something I would propose at all. You can go round and round with this. One thing is certain though, succession goes in one direction and never stops.

Over the last eight years I have recorded 73 species in the survey but this morning added another one to the list in the form of a pair of Egyptian Geese flying over. Whatever next?!

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