Microclimatic factors in avian breeding patterns: Implications for woodland nature reserve design.

Temporal variation in the first day of egg-laying of the Blue Tit within a deciduous woodland is accounted for by the small but significant spatial differences in microclimate, particularly temperatures. The warmer areas, which are confined to the southern half of the wood, are also characterised by larger clutch sizes and greater survival of nestlings into the local population. Furthermore, examination of the total breeding avifauna of this wood shows densities to be highest in the warmer areas and particularly along the south-facing edge zones. Investigation of 16 woodlands (range 0.01-14.9 ha) shows that bird species richness increases with increasing woodland area but that there is an inverse relationship between breeding bird density and woodland size. However, the total species number and breeding density of 14 small woods are markedly higher than those of a single large wood of roughly equal size. These differences are accounted for by the high proportion of edge zone within a group a small woodlands (86%) compared with the single larger wood (24%). The clear importance of small woodland patches to the total avifauna of East Leicestershire, and presumably other parts of lowland England, is stressed. The proportion of edge zone is influenced by woodland size, shape, internal structure (including the presence of rides and clearings), slope and aspect. Combinations of these factors can result in edge zone depth being considerably increased in a woodland. The importance of the edge zone to maximising species numbers is described in relation to various strategies of woodland nature reserve conservation. One such strategy advocates that species numbers and breeding densities in a very large wood can be enhanced by careful attention to management proposals such as ride-widening, tree-thinning and coppicing. The influence of these findings on future large-scale planting is also discussed.