|Site types that use this protocol||
Observations of the devastating effects of agricultural pesticides on bird populations during the late 1950s and 1960s strengthened the view that only objective, regular and scientific monitoring of birds can give an insight into the changing situation of different species. It was also realised that such monitoring can also reflect perturbations affecting the wider environment. Birds are relatively easy to observe and are thus good subjects for a monitoring programme. Moreover, the large number of bird species, over 200, which breed in the United Kingdom have different feeding patterns and occupy many different habitats, making it likely that at least some species will react to particular environmental changes of whatever type. It is well recognised that bird populations are affected by many man-induced factors in addition to pesticides and other agro-chemicals. Land use changes such as drainage and afforestation affect both nesting sites and food availability. Birds are also affected by variations in climate and in particular by periods of severe weather, either in the UK, for species over-wintering here, or in other countries for migratory species.