Maybe 48 hours wasn't a long enough rest? Then again the birds weren't that old... How did you kill them, since everyone is pointing as that as a possible answer? Killing cones and slitting the artery in the neck but leaving the windpipe intact will kill the bird a bit slowly and allows it to bleed out well. As others advised, ice water for cooling...as long as its 50 degrees its cold enough. And letting it sit in the fridge for a few days (I'd do more like 3). I know that in other animals, the slower they cool the more tender the meat because there's a chemical/enzyme or something the body releases that makes it more tender. I think with birds this is what is happening as we let it "age" in the fridge.
So, in short: How are you killing it? How did you process it? How did you chill it?
All things fail, I'd let them sit 3 or 4 days in the fridge next time, just to be sure.