A yardstick for measuring populations of small rodents

(1) Six-monthly trappings (summer and winter) were done between 1952 and 1969 of Wood mice (Apodemus sylvaticus) and Bank voles (Clethrionomys glareolus) in Wytham Woods, near Oxford. One hundred and fifty Longworth live traps were used on two 2-acre grids in oak-ash-sycamore woodland. (2) The traps were pre-baited for 48 hours and then set. The catch was examined on the evening of the same day and on the following morning. The total of animals caught during the 24 hours was used as an index to the actual numbers present. Animals retrapped in the morning (identified by a simple method of marking) were omitted from the total. (3) Between 1969 and 1972 the same procedure was used but trapping, evening and morning, was continued for 3-4more days until some 75–80% of the catch consisted of marked animals. (4) This enabled estimates to be made of the actual numbers of voles and mice present at each trapping and so to assess the proportion of the total present which was caught ring the first 24 hours. (5) These estimates were made by (Method la) a straight Lincoln Index, (Method lb) a Lincoln Index method as modified by Hayne (1949) and (Method 2) by Hayne's trap-out method. Over six trappings the estimates by these methods showed reasonable agreement. (6) Comparison of the index trapping (i.e. the numbers caught during the first 24 hours) with the estimates of the total numbers present showed that, on all occasions, more than half of the mice and voles present entered the traps during the first 24 hours. With Bank voles there was a tendency for about half of them to be trapped during this time during the winter and about three-quarters of them during the summer. (7) With Wood mice the figures are more variable but, again, never less than half were trapped during the first 24 hours and, on some occasions, nearly the whole of the population. (8) These results show that reasonably accurate estimates of the numbers of these rodents can be achieved within 3–4 days' trapping (estimates that can be cross-checked by different methods) by using a high density of traps (see (1) above). Furthermore, trapping for 24 hours only will always catch at least half of the population present and, on occasions, considerably more than half.