I was about to take a picture but it had already disappeared.
I was about to capture an image of the bird but it was gone.
I was about to take a shot of the bird but it was not there.
"出てきたと思ってカメラを構える隙にもう潜っちゃった" は、
I was about to take a picture but it had already disappeared.
I was about to capture an image of the bird but it was gone.
I was about to take a shot of the bird but it was not there.
などが言いやすいかと思います。
どれぐらい詳しく言うかにもよるのですが、
私も鳥や小動物の写真を撮るのが好きで、よくこういう状況になるので、上のような感じで言っています。