I think there needs to be a balance in TNR. Some people are so enthusiastic to "live and let live" that they do indeed let sick and older cats stay in the colony...which increases competition for food, territory, shelter and other resources. So a huge colony is struggling to survive.
On the other hand, I believe that sick, injured and very old cats with arthritis should be humanely euthanized. This lets the colony be smaller with healthy and/or younger cats, so that they thrive
, rather than just merely scrapping by every day for necessities. IF medical care can be given to sick/injured cats, by all means...let them stay. But if you can't handle them to administer medicine, change bandages or other care, there's no point in letting them suffer simply because somebody has a strictly "no kill" mentality.
In my opinion, if they are fed enough food from humans, they won't hunt as many birds. They'll still hunt some, as it's instinct, but not purely out of hunger like if they were totally neglected.
Right now in my yard, the plum tree is FULL of little birds (they look kind of like chick-a-dees but aren't). There's also plenty of starlings, robins, tanagers, western kingbirds and in warm weather I've seen at least a dozen hummingbirds on the rose bush. Yes the cats get some sometimes, but I doubt the birds are in any danger of being totally wiped out. And of course the magpies...who steal the cat food a lot of times
The colony I take care of started with 13 cats. Due to many reasons including my budget for food, illnesses, attacks from other animals, etc. it is now down to 4...I think.
Zazzle I see every day. Tangelo I see a couple times a week. Disco has been missing since March 2011, and I haven't seen Rune since Jan. 2011.
People might get mad at me for putting down 10 cats. But the ones I now have are healthy, have plenty of food, and I have enough shelter for each cat to have their own little winter house (though they do share sometimes). I am much happier seeing this, than watching over a dozen cats limp around and "hack up a lung" all the time when they get sick every other month.
Oh, and I have rescued 2 litter of kittens (tried trapping their mom for TWO YEARS! She was trap-smart). Disco and Zinara I kept. 5 other kittens were tamed and adopted into Forever Homes. So I do try to save who I can!