Welsh Journals

ROOKERIES IN CARDIGANSHIRE A.O..M. & E. H.CHATER INTRODUCTION Between 1959 and 1967 we counted the nests in as many rookeries as we could find in Cardiganshire. The only previous survey of which there is published information was made by C. L. Walton in 1913-1916, and a very brief summary of his results is to be found in a paper by him (Walton 1928). We attempt below to compare his survey with the relevant part of ours. For the most part, however, we simply try to give an account of the size and distribution of rookeries in the county in the hope that our results may be useful in assessing any changes in the Rook population in the future. There are considerable difficulties in estimating how many nests are in use in a rookery in any one year. In the weeks immediately before breeding starts the number of nests may increase, as nests surviving from the previous year are rebuilt and new nests are constructed; or the number may decrease, as old nests may be dismantled in order to build or repair other nests, perhaps even in other rookeries. It is difficult to tell which nests are actually in use even at the height of the breeding season, nests in clusters may be difficult to count separately, and nests in evergreens may be vir- tually uncountable from the ground. Our counts were mostly made in late March and early April at about the time of laying. Later in spring, the foliage makes counting more difficult, and in winter the number of nests is often reduced to a quarter or even less. Rookeries often occur in groups, and for the purposes of counting we have considered one rookery to be separate from another if it is more than 200 metres away; colonies of nests closer to one another than that we have considered as only one rookery (Coombs, 1961, provides a much more ideal definition of a rookery which we were unable to adopt "I have regarded a divided rookery to be one unit if the 'nuptual' pursuit flights go round both parts and involve members of both parts"). The possible errors in our counts due to the above factors must therefore be kept in mind, but much more important is the fact that our counts were spread over a period of 9 years during which some rookeries were aban- doned, others were started afresh, and the number of nests in each one fluctuated from year to year. In the discussion which follows we try to compensate for this. Copies of our detailed list of rookeries, on which this paper is based.have been deposited at the National Library of Wales, Aberystwyth, at the Department of Zoology, National Museum of Wales, Cardiff, at the British Trust for Ornithology, Tring, and at the Edward Grey Institute of Field Ornithology, Oxford. Each entry in the list consists of a 6-figure grid reference, a brief description of the locality and its height above sea-level. the kind of tree in which the nests were built (if this was recorded), the dates on which counts were made and the number of nests seen. North and east of a line from New Quay to Lampeter we probably missed few rookeries and the coverage of this part of the county can be considered reasonably complete. The coverage of the remaining quarter of the county is probably not so thorough, but we include it nevertheless in our statistics.