The Population of Rooks (Corvus frugilegus) in West Gloucestershire. II

Abstract

I936 fall is therefore probably not significant, that of I938 may be. An attempt has been made to relate this fall to other factors. The numbers of nests have been compared with the Ministry of Agriculture and Fisheries' returns for 4 June of the previous year, proportionate adjustments being made to allow -for the fact that the area under survey does not exactly follow the parish boundaries. One anomaly was immediately apparent; between 4 June I932 and 4 June I933 the total area of agricultural land (crops and grass) fell by about I7%; it remained sensibly constant until I936, and then in I937 rose to about its I932 value, where it remained. This cannot mean that there was any real change of land use, but must indicate some changing method of filling up the Ministry's forms. No figures are therefore given for this factor. Acreage of cereals, of cereals; roots and potatoes; and of total arable land varied irregularly from year to year, though the three factors generally moved in the same direction. There is, in general, no connexion between the movements in the acreage of arable and the rook population, but the years I936 and I937 are I35 % and 8-8% respectively below the average in acreage of cereals. It is possible that this was a partial cause of the fall in population in I938. The cereal acreage in I938 was a little above average. Further reference to this point is made below. During the war of I939-45 there was much ploughing of grassland, and in I943 the acreage of cereals was about seven times, and that of total arable about five times, those of the years I933-8. The significant, but not large, increase in the number