Wheatears in the Presellis: an attempt to estimate the population

I was prompted by my Atlas Conscience (aka Bob Haycock) a few weeks ago, when we bumped into each other on the Deer Park. “Have you done your last TTV yet, or do you want me to do it?” Awful guilt washed over me, and the very next day I set out with a friend who was staying with us to go up to the Presellis. The Tetrad that I had volunteered to do several years ago is SN13L, the very easternmost part of the Presellis, with a fascinating range of habitats, from the stunning Bronze Age fort and open moors of Foel Drygarn to the plantation and clearfell of Llethermawr. My last three TTVs have been utterly enjoyable, and have given me some great Pembrokeshire bird experiences, often so very different from the Marloes peninsular and the Islands.

The highlight of this particular TTV was probably a group of very young Redpolls in the plantation. But we had decided to make a day of it, and walk the length of the Presellis, ending up with a pint at the wonderful pub in Pontfaen. What struck me, as we walked along the ridge, was not just the incredible number of Wheatears that we saw (9 in the first hour of the TTV, 15 in the second, and over 70 in a 10km stretch of the walk over generally suitable moorland), but how predictable they were. Every significant outcrop of rocks that we passed produced at least a couple of birds, and often half a dozen or more. Whereas the path across the open grassland and bog was devoid of the birds (although with plenty of Meadow Pipits). The Wheatears, whether adults or fledglings, seemed very reluctant to move far from the rocky outcrops, either. My thesis, therefore, was that we could estimate the population of Wheatears in this area by estimating the number or area of the hills that are rock outcrops, as opposed to grass and moor.

Interestingly, the 1:25,000 Ordnance Survey map plots such outcrops relatively accurately, and I have used this as a first step. It shows the number of significant outcrops as 90-100 in the main area of the Presellis (essentially Foel Drygarn to Carrig Lladron), and another 80+ on Carn Ingli and the outer/more isolated hill areas. Given that some of these outcrops and disused quarries can be several hectares in area, it is possible that these could support several pairs of Wheatears. I would suggest, therefore, that the Presellis could support a breeding population of at least 150 pairs of Wheatears. This is compatible with the most recent estimate in the Atlas of Breeding Birds in Pembrokeshire 2003-07 of 400 pairs for the County, and equates to 5 pairs per Tetrad where the birds were found in the Presellis. Given the high densities on Ramsey, in particularly (100+ pairs) it still would suggest only around 150 pairs for the other Islands and the entire distance of the Coast Path, which feels a bit low to me. It may be that 2011 has been an exceptionally good breeding year for these lovely birds, and my walk took place at a time of very good visibility of the young birds. I would be interested, however, to hear of anyone else who might have encountered such high densities of Wheatears elsewhere in the Presellis, or who believes that other areas have significantly lower densities.