ANNUAL VARIATION IN BIRDS AND PLANTS OF A TROPICAL 2ND-GROWTH WOODLAND

We examined effects of successional changes in vegetation on bird spec ies found in a young second-growth woodland in Costa Rica over a four- year period (1985-1988). Significant changes in vegetation structure a nd plant composition occurred: numbers of small (2-5 cm dbh) trees, wo ody shrubs, and lianas increased, as did canopy cover; herbaceous shru bs declined in abundance. Total numbers and species of plants producin g flowers pollinated by hummingbirds or fruits eaten by birds did not change, but significant changes were noted in fruit production of indi vidual plant species. Overall capture rates of birds in mist nets did not vary significantly among years but significant changes were noted for individual species. Several bird species characteristic of mature forest habitat first appeared in second-growth woodland during latter years of the study; other species, more dependent on early successiona l habitats, decreased in abundance or disappeared entirely from our st udy plot. Diets of some fruit-eating birds changed over time, apparent ly in response to changes in abundance of different fruit species. Thi s study suggests that changes in plant and bird populations may be rel atively rapid in tropical second-growth habitats. Such results have im portant implications for conservation; a range of successional stages may be necessary to support the full complement of bird species that c an occur in a given area.