Minimum summer populations and densities of bats in N.E. Scotland, near the northern borders of their distributions



The contemporary summer populations of three species of bats, the common pipistrelle (Pipistrellus pipistrellus), the brown long-eared bat (Plecorus auritus) and Daubenton's bat (Myotis daubentonii) in an area of 3200 km2. in N.E. Scotland (57°N), near to the northern borders of their distributions, were assessed by surveying nursery roosts. A total of 184 roosts were discovered between 1973 and 1989, and the species in occupation was identified at 147 of these. Of the roosts with identified occupants. 109 were occupied by pipistrelles. Most pipistrelle roosts were located close to the major rivers. The mean roost size in early summer for pipistrelles was 117 individuals. The larger roosts were predominantly (98.8%) female (n= 165). Captured bats foraging on the rivers were only 67%) female (n= 102) which indicated there was also a large population ofmales which the roost survey did not detect. The minimum density of females in the occupied habitat was estimated at 18.2 bats per km2. This contemporary population in the study area is much greater than was indicated in historical records from the 19th century. There was no evidence that the populations of pipistrelles, near to the northern border of their distribution. was reduced when compared with populations further south.

Thirty-four of the roosts were occupied by brown long-eared bats. These roosts were located predominantly along a short(25 km). well wooded section of the River Dee. The mean roost size in early summer for brown long-eared bats was 16.8 individuals. On average, these roosts were predominantly female(69.6%, n = 273), but contained a variable proportion of immature and occasionally mature males. The density of brown long-eared bats in the occupied habitat was 1.66 bats per km2, less than a tenth the density of pipistrelles. Historically, this bat was more widespread and abundant in the area.

There were only four identified roosts of Daubenton's bats, containing in total about 300 individuals. In all these cases, the roosts were located immediately adjacent to the major rivers, and in two of the cases were also close to areas of open water. A survey of 72 bridges in the area revealed only two individual Daubenton's bats. The density of Daubenton's bats in the occupied habitat was around2–4 bats per km2. There has been a substantial reduction in the population of this bat in the area over historical times. At the turn of the century it was the commonest bat in the area, but currently it is the rarest. The causes of the decline are uncertain but are unlikely to reflect draining of areas of open water or the activities of anglers.