I encourage you to read this, from the Humane Society's web site.
http://files.hsus.org/web-files/PDF/PUBS_Slater2.pdfWith a well funded and staffed TNR + management program, and removal & adoption of adoptable ferals from the colony, the number of feral cats may finally begin to be reduced. Isn't that the common ground for everyone?
I have volunteered for a TNR, it was a blessing for those cats...to be alive yet not struggling with heats, pregnancies, nursing, fighting for territory, starvation, etc...
What is the alternative that does not include mass euthanasia? (and that 'solution' hasn't worked yet, the numbers just keep increasing) Eventually, if everyone did TNR with feral colonies, the numbers would dwindle, and your birds would have less threats. Please let me know if there is another alternative that works as well or better, because I will support it!
BTW- edited to add, I am a big advocate of city cats staying indoors, they don't need to run into a feral colony that hasn't been fixed yet!