One more page and you will know more than most about the Textural Discontinuity Hypothesis.  This one is pretty simple, the height of each green cylinder represents the body mass of that particular bird species.  The body mass of a species is strongly correlated with stuff like, habitat usage, food usage, and life span.  If the bird species has a greater body mass it uses larger resources.  The transition zone between resource clusters however are hypothesized to be kind of fuzzy.  This mean that resources at the edges of resource clusters are more variable in their availability then are resources towards the center of resource clusters, so birds that use these resources don't have a reliable source of resources to depend on.