What was done
The authors, as they describe it, "integrated the exposure of species to climate and land-use change through the combined effects of these drivers on global land cover and explored the resulting reductions in range size and possible extinctions within the world's 8,750 terrestrial bird species," using the simplifying assumption of stationary geographic ranges that allowed them "to quantify risk in terms of the projected vegetation changes across a species' current range." This assumption, as they readily admit, "yields worst-case projections," but it provides a "first baseline assessment" of the relative strengths of the negative impacts of projected global warming and anthropogenic land-use changes on bird habitats worldwide.

What was learned
Although Jetz et al.'s simplistic analysis suggests that "expected climate change effects at high latitudes are significant," they write that "species most at risk are predominantly narrow-ranged and endemic to the tropics, where projected range contractions are driven by anthropogenic land conversions [our italics]," and they add that this "habitat loss in economically emerging tropical countries will continue to pose an even more direct and immediate threat to a greater number of bird species," such that "even the most optimistic scenarios lead to substantial range contractions of species, especially of those already vulnerable to extinction because of their current restricted ranges."

What it means
Far greater harm to earth's many bird species will likely be caused by man's destruction of natural ecosystems -- in order to use the land and water resources that have historically sustained those ecosystems for the growing of food crops (needed to sustain our growing numbers) and biofuels (wrongly believed to be effective in fighting rising temperatures) -- than will likely be caused by any anthropogenic-induced global warming; and this phenomenon will likely be most strongly expressed in "economically emerging tropical countries," which can least afford to bear the catastrophic negative consequences.