One of the side effects of using long lenses at close and even moderate ranges is that depth of field leads to considerable blurring of the foreground and background. This photo demonstrates it nicely. I normally use it to isolate my subjects from the background and eliminate mesh/bars in the foreground. I shoot with a wide-open lens to accentuate the effect. Unfortunately, that make focus even more critical than normal.

This is a more extreme example of what happens with your eyes. Focus on something very close, and everything else goes blurry. It is also why you subconsciously expect to see some level of blurriness in the background or foreground of artwork.

I am missing the sign photo from this exhibit. If my memory is corect, they did not have one. So I cannot tell you what birds are in this photo.