3.3 Inventory and distribution studies The great majority of species records are collected at a county level. Records are generally collected on a casual basis overtime and are derived from roost visits at the request of householders, reports of dead, trapped, injured or grounded bats, reports from the public, from bat walks or from specific roost, feeding area, bat box or hibernation site surveys. Plotting records collected in this way by some map unit (e.g. km squares, gives an indication of the distribution of species, but care should betaken in interpreting such maps. There are inherent biases in the ability to find some species. House-dwelling species, such as pipistrelles, are more likely to be located than tree-dwelling species, such as noctules. In surveys of free-flying bats, loud echolocators, such as serotines, are more likely to be located than weak echolocators, such as Natterer’s bats. If coverage of an area is uneven, maps may show the distribution of observer effort as much as the distribution of bats. A more accurate picture of distribution can be gained if a specific atlas survey is organised, which is designed to ensure a fairly uniform coverage indifferent areas, and uses standard searching and species identification checking procedures. Failing this, coverage and search effort need to be measured and reported. A good example of an atlas study at a national scale is one completed in the Netherlands (Limpens et al., 1997).