Corvus monedula – JAC Y DO – Breeding resident
Comparison with previous atlas:
|No of tetrads occupied||449 (of 478)||437 (of 490)|
|Percentage of tetrads||93.9%||89.2%|
At the close of the survey of 1984-88 it was realized that the only assessment of breeding Jackdaw numbers anywhere in the county came from the islands of Skokholm and Skomer. An estimate of the population for the whole county had not previously been attempted. An assessment that an average of 20–30 pairs per tetrad was possible was used, postulating that there were 9,000–13,000 pairs breeding in Pembrokeshire. The 1988-91 National Atlas calculated an average density across the UK which was a little lower than the lowest value used for the initial local survey. The National Atlas also assessed relative abundance, the map of which showed Pembrokeshire at maximum. Making an adjustment to allow for this density suggests that the original estimate of 20 pairs per tetrad in the county was realistic. The BBS then indicated a 36%increase in Wales as whole between 1994 and 2007. No evidence was found that such an increase had taken place in Pembrokeshire but some sign that it has not.
Over the years there has been a 33% decline in the number breeding at Skokholm and a 66% decrease at Skomer. Studies on the islands have shown very low productivity even during times when breeding numbers increased there, meaning birds were being recruited from the mainland. The declining numbers which followed suggests such recruitment ceased, probably because there was no longer a surplus of birds on the mainland. With this in mind it seems best to be cautious and consider that county numbers had not increased. In this case the breeding population in Pembrokeshire at the end of 2007 was thought to have been about 9,000 pairs.