THE EFFECT OF OBSERVERS VISITING THE NESTS OF PASSERINES - AN EXPERIMENTAL-STUDY

The effect of visits to nests of open-nesting passerines was investiga ted over two years using a randomized sampling design; the only such s tudy ever undertaken in the UK. There was no overall effect of nest vi siting on nesting success (the analysis had a power of 85% to detect a difference of 10% in success rates between control and visited nests) . However, although thrushes and insectivores appeared to be beneficia lly affected by nest visiting, finches were detrimentally affected by nest visiting in one of the years. The effect of nest visiting did not differ between habitat, categorized into farmland, woodland and garde n. The degree of exposure of the nest had no effect on nesting success bur increases in the degree of disturbance made to a nest's surroundi ngs paradoxically appeared to increase nesting success. A brief review of studies undertaken outside the UK also reveals little evidence for any effect of nest visiting on passerine nest success.