The answer to the obvious question

  

Here are the short-billed dowitchers.

 

If these are the short-billed, how long is the bill on a long-billed dowitcher?

 

Here are long-billed dowitchers.

 

Good luck telling the difference.

 

There are some subtle physical differences.  Long-billed is a little chunkier, but the best clue is habitat.  Short-billed prefers salt water.  Long-billed prefers fresh water.  Mostly.