Mike,You will never get a clear definition on knife terms.They have evolved over ages to be what the maker wants to call them.It would be a perfect world if there was an ASA for knife terms.Look at the range of things called "Bowie Knife".
A trout and bird knife is a small fillet style knife.They are light,thin,and often flexible.Bird knives tent to be smaller than trout knives and have stiffer blades.The handles are simple,usually without guards.They are designed to dress small game,and are thus using knives.It tends to be about 3-4" blade length,but the trout fillet knives I sell have about 5" blades.
Of course, this is my definition,hope it helps.
Stacy