Density-dependent habitat distribution in birds: issues of scale, habitat definition and habitat availability

For many species, distribution across different habitats may be a dynamic p rocess affected by population size and the position of individual habitats on the continuum of habitat quality from most preferred to least preferred. Species that exhibit density-dependent habitat preferences should show a n egative relationship between the proportion of a population in a particular habitat and total population size over time in those habitats that are rel atively preferred, and positive relationships in habitats that are relative ly less preferred. Such relationships were analysed within 13 farmland stud y areas for 12 bird species. At least one significant relationship was dete cted per species, but no species showed consistent significant relationship s in any habitat across plots. Furthermore, both negative (relatively prefe rred habitat) and positive (relatively less preferred habitat) relationship s were detected in different study areas for a particular species in a give n habitat. When habitat variation across study areas was controlled in a co mbined analysis, only four species showed strong evidence of density-depend ent habitat use. The lack of stronger relationships between population size and habitat use is likely to be a consequence of defining habitat at too c rude a scale. Density-dependent habitat processes may be more usefully exam ined when specific micro-habitat features that contribute to territory qual ity can be identified. Inability to detect consistent patterns of habitat p reference may be caused by spatial variation in habitat availability, habit at quality and inter-specific competition.