Jack, I certainly can't disagree. The same thing's true of street photography. You can't exactly plunk down a tripod and set your camera on mirror-up.
I can see how that would hold true also ...
On the other hand, I think that with enough practice you can learn to frame very quickly.
Agreed. More than that, just as the true pro in nature photography will learn all the nuances of his environment well enough to position himself
to get a good shot ... even before his intended subjects arrive ... I would imagine this would also be true of a street photographer. Thus, rather than being "surprised" by the moment, they are prepared for it.
Here are a couple examples of the kind of "wild animal" shot you're talking about. Neither of these is cropped. In the spirit of full disclosure I do have to admit I've missed a lot of these, but they all were clean misses. None of them were situations where cropping would have helped.
I like the first image quite a bit, and there is enough interesting detail in the wings, the light, and other elements of the capture to make me very forgiving of some of the secondary (and less interesting) elements of the watery background. Cropped, I would think it would be even more compelling.
The second image is just okay. It's a bit too close, and the background too ordinary, to inspire me a lot ... so I agree, no cropping would help this image become extraordinary.
However, with the first image, while not perfect it still demands that someone stop and look at it longer than usual ... and I would imagine more people would be inclined to "chimp" over it (ooh!-ooh!) than not