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THE BREEDING OF JACKDAWS ON SKOKHOLM A. S. RICHFORD AND D. M. LAWMAN INTRODUCTION The history of Jackdaws as a breeding species on the islands of South Pembrokeshire has been one of expansion and colonisation. Before 1946 only a few pairs bred on Skomer. However, between 1946 and 1964 the breeding population rose rapidly to 200 pairs and has remained at that level since (Birkhead, 1974). In 1965, just after the stabilization of numbers on Skomer, Jackdaws were first recorded breeding on Skokholm and here also the population increased rapidly (Lawman, 1974). In 1974, 55 pairs bred and no further increase has been observed. It is interesting to note that the breeding density of Jackdaws on both islands is now roughly equal at about 1.5-2 pairs per hectare. The Jackdaws breed in small colonies, nesting in rock crevices and in disused Rabbit burrows on the grassy slopes above the cliff edge. A number of nests on Skokholm were visited regularly in 1975 and 1976 and information on chick growth and survival was collected. Because of the inaccessibility of many of the nests only eight were studied in 1975 and nine in 1976. However, this sample is probably representative since the data are comparable with those collected in a more detailed study on Skomer, where similar conditions prevail. The food of the nestling Jackdaws was sampled using the collar method (Kluyver, 1933). BREEDING BIOLOGY AND SUCCESS Jackdaws lay their eggs at daily intervals and for the purposes of this study the laying date of any clutch is defined as the date on which the first egg of that clutch was laid. The median laying dates for the populations of Skomer and Skokholm in 1975 and 1976 are shown in Table 1. Jackdaws on Skomer laid significantly earlier in 1976 than in 1975 (P < 0.001) and although the difference is not significant a similar trend is apparent in the Skokholm data. The differences between islands are not significant in either year. These laying dates are well within the normal range for Jackdaws in Britain (Holyoak, 1967). TABLE 1 Timing of breeding of Jackdaws on Skokholm and Skomer. Year Median Laying Date Range Sample Size SKOKHOLM 1975 22 April 22-31 April 8 1976 20 April 18-26 April 9 SKOMER 1975 27 April 18 April-12 May 75 1976 23 April 16 April-2 May 92