Vertikale Stratifizierung und die Benutzung der Vegetation und der Lichthabitate von südamerikanischen Waldvögeln

The vertical stratification of bird species has been intensively studied in both temperate and tropical bird communities. I investigated the vertical stratification of bird species and their use of vegetation structures and light habitats using observation data from 92 rainforest species. Most observations were made from a 40 m high canopy crane situated close to the Surumoni river (65°40′W, 3°10′N) near Esmeralda, Estado Amazonas, southern Venezuela. Further observations were made at several forest sites nearby. Previous analyses had shown that bird species foraged in stratified vertical niches, with wider niches for midstorey birds than for either canopy or understorey birds. In this study, I used across-species as well as phylogenetically-controlled analyses to correlate the vertical position of bird species with variables relating to vegetation structures and the amount of illumination. I found that species which foraged low in the forest were mostly found in small gap and forest shade light habitats, while species that foraged high were mostly found in large gap and woodland shade light habitats. Furthermore it was the lower-foraging species that were more often found in dimmer light conditions inside denser cover. In correspondence with their wider vertical niches, midstorey birds foraged in a wider range of light habitats and illumination than either canopy or understorey birds. While preserving their overall stratification, species actively moved within their preferred stratum, e.g. downwards and into denser vegetation during hours of strong sunlight and around midday when temperatures are highest. Bird activity appeared to be lowest shortly after midday, as I discovered when controlling for the effects of chance encounters with multi-species flocks. These observations are important not only for the determination of foraging niches, but also for the study of crypsis and the conspicuousness of a bird species in relation to its foraging stratum.