C.S. Robbins R.F. Whitcomb 1977 Island biogeography concepts applied to breeding bird populations of woodlots clearly explain why certain passerine species typical of tracts of 1 km2 or more are absent from tracts one-half or one-quarter that size. Many 'natural areas' now being preserved are too small to support the complete complement of nesting species; this is partly because little knowledge of the minimum areas required is available to the agencies and individuals making the decisions application/pdf en Application of census techniques to habitat preservation strategy article