Seasonal variation in prey composition and digestion in small mammal predator assemblages

Many aspects of taphonomy have a time dimension which affects the way modifications develop. Weathering is an obvious aspect of this, and for this there is an added aspect of variations by geography or latitude. A long-term taphonomy project in Wales was set up to investigate how small mammal assemblages may have changed in relation to seasonal and climatic variation. A collection of 304 pellets produced by a single pair of tawny owls (Strix aluco) spanned 40months from 1982 to 1986, and pellets were collected from the roosting site on a monthly basis. The project ended in March 1986, when the owls left their roosting place, with a follow-up collection from the same site but from a different owl individual in 1987. Both the species composition of the owls' prey, and the digestion of their prey, differed from month to month, with the major differences being between summer and winter and variations in the local climate. Earthworms were the most abundant prey, but they were more abundant during the summer months and during mild winters. Microtus agrestis was the most abundant mammal species in the pellets and the next most common prey species, closely followed by Apodemus and a large beetle species. Digestion of arvicolid teeth and post-crania was greater during the winter months, but no difference was found for murid teeth between summer and winter collections because the summer sample was too small. Amphibian bones were also more digested in winter. Digestion of Apodemus molars was about half that of Microtus, but digestion of incisors was the same, and where both occurred in the same pellets they were on average one stage behind that of Microtus molars. It is concluded that seasonality and local climate are important factors both in the prey composition and the degrees of digestion of prey in tawny owl assemblages.