Thrush Predation on the Snail Cepaea hortensis

Abstract

In the summer of 1953 Dr R. A. Hinde kindly drew my attention to the thrushes which were feeding on snails on the tops of bird cages at the Madingley Field Station, near Cambridge, and it seemed that this might provide an excellent opportunity for studying the effects of predation on a natural population of snails. I examined the cages on 29 June 1953 and collected 123 dead thrush-eaten shells of Cepaea hortensis (Muiller) from their roofs. All dead shells were removed from the area, and regular collections were started in January 1954 and have been continued in an unbroken series at the end of every month for three years, until December 1956. Only shells with the upper part of the lip intact, where it joins the next whorl, were scored, so that the same shell could not be counted twice. The numbers of shells found in each month are shown in Fig. 1, and the different polymorphic shell types in Fig. 2 and Table 1. All the shells were collected from a row of twenty cages, evenly spaced in a straight line for 140 yd along the eastern edge of a narrow strip of woodland, just inside the boundary fence of the Field Station. The cages were all the same, measuring 6 x 6 x 6 ft and made of wire netting on a steel frame. Each cage was partially roofed with a piece of asbestos sheeting, 6 x 2 ft in size, and it was these roofs that were being used by the thrushes as anvils for breaking open the snail shells. In the winter the shells were usually opened on the frozen ground close to the edges of the cages and an area extending 3 ft out around the bottom of each cage was also searched. The birds seemed to prefer to open the snails close to the cages and not many extra shells would have been found more than 3 ft from the cages. These well-defined and relatively small areas could be searched quite intensively, and a repeat search showed that very few of the shells in the specified area were being missed. The rest of the wood was carefully searched on several occasions, and some shells opened on tree-stumps well away from the cages were found, but it did appear that the cages were the main sites of the anvils used by the birds for opening snail shells. The only birds that were definitely seen feeding on snails were song thrushes, Turdubs ericetoyum Turton, but there were also missel thrushes, T. viscivorus L., in the wood and possibly other species as well in the winter. Apart from a few Theba cantiana (Mont.) the only thrush-eaten shells found were Cepaea hortensis; no C. nemoralis (L.) were found. The C. hortensis population in the wood did not appear to be a particularly abundant one. Practically all the shells seen on the anvils were fully grown or nearly so; younger snails may also be