The use of different stages of a secondary succession by birds

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Abstract
The bird species utilizing experimental field sites, representing 3 stages in a secondary succession, are described in terms of species richness and biomass. While species richness increases with successional age and was highest in the Woodland site, bird biomass was greatest in the Young Field (an area of recently harrowed ground on which the natural vegetation was allowed to develop). The total bird biomass on each site, weighted by the amount of time spent by each species on a site, shows the site utilization of the Old Field (an area of permanent grassland) to be low. The small mammals were the dominant vertebrates in this site and appear to compensate for the low biomass of birds since the total vertebrate biomass was fairly constant for the three stages in succession. The proportion of this biomass contributed by herbivores fell during succession. Conversely, the density of insects and the biomass of insectivores rose. It is argued that the higher biomass of vertebrate herbivores is a reflection of a simpler ecosystem early in succession.