Hey, Ellen...
It's definitely Blue's nature to do what he did, and no matter what you do to him, his nature will not change. He is a fine predator - no doubt even an impeccable predator - and we have to remember not to impose our humanform morals & beliefs onto another species. I knew of a fellow once who tried to make his cat a vegetarian because he himself loathed killing other animals. The result was that the cat became sick and almost died before the guy finally realized that it is the nature of nature to prey on itself; and that's just the way power has set things up.
The other thing that makes a world of sense to me is the "theory" (to me I would say it's a fact) that when it is a creature's time to die, the most noble death it can have is to give itself in service to another creature. Who's to say the bird wasn't injured, or would have been killed in some other way that same afternoon? To present herself to Blue was a gift the bird gave to the cat. Yes, it's brutal to our humanform way of thinking; but to Blue and the bird, it is a pact of spirit and I have no doubt that he was "bound" by that pact to do precisely what he did.
This morning, the ancient weenie dog we found running down the middle of the road about 2.5 years ago passed away. She was eating and active yesterday morning, then by last night, she had slipped into what amounted to a fitful coma - old age and infirmity having taken its toll. This morning, as she lay dying in my arms, I thought about the night we found her, and I wondered if I had done "the right thing" by plucking her out of the middle of a busy highway. There is no doubt in my mind she would have died that night - if not by being run over by a car, then certainly food for the local coyotes. And, if she'd been hit by a car, it's such a rural area she would have become food for the ravens and other scavengers. As ugly as it may have been to my humanform sensibilities, I couldn't help wondering if I had given her a "better" life, or deprived her of her noble death.
So... maybe it would help to consider the possibility that we all have an appointment with our destiny. We can either keep that appointment or not, depending on our own will, of course. Some days, we'll fly away and emerge victorious. But one day, Death will tap us just as it tapped the bird, and no matter what we do, it will be our time to dance with da cat.
Just me, but I think I might consider seeing things from Blue's perspective. He's just being the best warrior he can be, working with the nature given to him by Nature.
Love him for who he is. And thank him with all your heart for what he has shown you.
*warm embrace*
D