Question about the sharpnes and the subject size

[Rosl Roessner]

I really have a question for all the experienced photographers here.

I want to know, if there is any kind of rule, what size in the frame a subject needs to have to be sharp .
This means: if I have a small bird in a far distance and can not get closer when is it worth to make the image? If I have to make a heavy crop to have the bird big enough in the final image, what is the maximum to reach enough quality?

If I take a 100% crop for example, it does not look good when printed. If I have 25% crop, it will work well, when I look at it on the sceen or in a not too large print.
I know, that it depends, what I want to do with the image (slideshow, print, web...).
But I want to know what you think, when is it worth to press the shutter?
When I look through the view finder, what size should the bird have in the frame to get a good result?

The image I show here, is a typical image for my question. It is a small Tree-Creeper and although I used the 500 mm lens (eventually with TC) the bird is so small in the frame, that it is not possible to get a good final image.

I hope, that you can understand my question (and my english...) and can give me any advice?
Thank you a lot,
Rosl

[Axel Hildebrandt]

If it is a rare bird or one I haven't photographed, I would take my chances anytime. For web presentation you can get away with a relatively heavy crop, not so for prints. Some people think 50% of the frame is the limit. In the end it really depends what you want to do with it. If the light and exposure are good you might have a better chance to make a somewhat usable image.

[Alfred Forns]

Hi Rosl I would go with the flow and keep him small in frame but place him in one of the corners !!! Heavy crops are always tough

For this little guy might have gone vertical with lots of room on top placing the bird on the lower left corner !!

[Oscar Zangroniz]

Beautiful bird and love the background. Your questions are better answered by the experts like Al and Axel.
Congratulations.

[Rosl Roessner]

Thank you for your answers so far.
I just want to mention, that it is not about this special pictures. (The whites are blown out too...). I just took that picture to demonstrate my problems with the objects size.

[Jim Poor]

I think with these opportunities you have to look for unique compositions. Something like this

Or possibly like this:

http://www.pbase.com/jimpoor/image/93212683