Results are in from the city of Portland's spring 2012 Ecoroof Bird Monitoring assessment report (pdf).
That's right. You can breathe now.
Two Bureau of Environmental Services workers, along with an Audobon Society employee and 10 volunteers, kept tabs on nine locations around the city this spring.
Three were buildings with ecoroofs—roofs with plants on them, basically—three were buildings with conventional roofs (the so-called "control" group) and three were "ground-level greenspaces (mostly open with trees and some shrubs)."
Their findings: Birds were observed more often on ecoroofs than conventional roofs.
Which is to say, birds prefer plants to concrete.
The charts below, pulled from the report, go into greater detail.
For instance: On two occasions, birds were observed copulating on the ecoroofs. No such scenes were observed on conventional roofs, even though the risk of skinned knees is substantially lower with birds than with people.
The same pattern was observed with nesting, although the report does not make clear whether the same birds were involved, or what species they might have been.
Finally, the report's "lessons learned" section includes the following words of wisdom:
Being on rooftops with binoculars can cause people outside the study concern.