What are the legendary bird's catch rate?

My brother and I were playing, and we decided to challenge ourselves by catching the birds with a premier ball. We gave articuno a razzberry, I missed my throw, he hit it without even getting a nice throw, and caught it first try. I went on to try to catch zapdos and moltres, and it took upwards of 100 premier balls each. So what exactly are the catch rates for the birds in this game?