Those who have posted links so far, in my opinion,
have the right idea about what constitutes a good
and useful trout and bird knife.
Size is critically important. So many knives being offered are
way too large for the stated job.
The above links are to sites/knives that appear to fall within the
parameters of knives which will do the job.
Back in the seventies, I bought a Randall model 8 trout and bird knife.
It is much too large to use on grouse, quail, dove, and any fish I normally
hope to catch. I still have that knife, but it serves other functions.
Look for a fairly short, fairly thin, very sharp blade.
Actually, a three inch paring knife of good quality would do just fine.