Do assembly rules for bird communities operate in small, fragmented woodlands in an agricultural landscape?

The favoured state approachsensuFox (1987) was used to investigate the existence of assembly rules for woodland bird communities in an agricultural landscape. When birds were classified according to gross breeding habitat requirements, year-round resident, 'true' woodland species showed an excess of favoured states suggesting a possible assembly rule. There was weaker evidence for a similar assembly rule governed by foraging requirements. This pattern was shown for all woods together, and for most categories of woods, grouped according to size, shape or size and shape together. Summer migrants did not show such patterns, and their arrival appeared to mask any patterns established by year-round resident species. The statistical significance of the excess of favoured states was highest in 1990, when bird population densities were considerably higher than in 1991 and 1992. Interspecific competition appears to be a factor in structuring woodland bird communities within the area sampled. Some implications for the action of these assembly rules on the results of further habitat fragmentation are discussed.