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| Vertical stratification of bird species has been intensively studied in temperate
as well as tropical forest bird communities. One question that has received less attention is
the variation of foraging stratum breadth. Hypotheses trying to explain that variation are
based on differences in perception, evasion, adaptation, competition, and environmental and
geometric boundary constraints. Although some predict that foraging strata near the ground
should be narrower than those near the canopy, others predict that strata in the midstory
should be broadest. These hypotheses were tested using observational data of 92 rainforest
bird species. Observations were made from or near a 40 m high canopy crane situated close
to the Surumoni River (38109N, 658409W) near Esmeralda, Estado Amazonas, southern Venezuela
(Surumoni-Crane-Project). I found a highly significant parabolic relationship between
mean foraging height and its standard deviation showing that midstory bird species
foraged in a broader stratum than either understory or canopy species. Using a conservative
comparative method, I also showed that the relationship was not the result of the phylogenetic
relatedness of the species involved. Analysis of previously published data supports
the relationship for two other tropical avifaunas. Because the ground and the outer canopy
represent geometric boundaries to themovement ofmost forest birds, two simulationmodels
were used to test whether midstory species may simply have a broader stratum because they
are less constrained in their movements than species near the boundaries. The ‘‘random
model’’ allowed individuals to move randomly between the two boundaries. It illustrated
that boundary constraints alone cannot explain the observed parabolic relationship. In the
‘‘preference model,’’ individuals tended to move back to their preferred height within the
forest column. That model simulated the observed parabolic relationship once a sufficient
number of time steps had been reached, suggesting that a simple combination of geometric
boundaries and preference for a specific foraging height may generate narrower strata near
the boundaries. The other hypotheses only explain the parabolic relationship if other factors
(e.g. vegetation density) are vertically distributed in such a way as to generate the observed
parabolic pattern. Finally, I briefly discuss the advantages and disadvantages of using raw
data in across-species analyses versus corrected data in phylogenetically-controlled analyses. | |
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