Birds and cities are like grape juice and paper towels

In TV advertisements, it seems paper towels can always absorb more grape juice.  Are urban areas similarly able to take in more and more introduced bird species? Does this species absorption have a limit? In this episode, Willson and Ellie don't know, can't agree, and so speculate wildly while discussing the study "Alien species richness is currently unbounded in all but the most urbanized bird communities" (Tsang, Dyer & Bonebrake 2019).