Talons are actually just feet with the tissue 'talon' applied (which is functionally the same as claw, tooth, ivory, nail, etc). So if you wanted just talons have the grasp flag and not all feet, you would need to make your own talons. Since feet are attached to bodies in the stock files, you'll actually need to make your own full body. It's easy though!
Here's how:
Make a new text file and name it body_murdoc.txt (or whatever). Put this in it:
body_murdoc
[OBJECT:BODY]
[BODY:HUMANOID_ARMLESS_GRASPTALONS]
[BP:UB:upper body:upper bodies][UPPERBODY][CATEGORY:BODY_UPPER]
[DEFAULT_RELSIZE:1000]
[BP:LB:lower body:lower bodies][CON:UB][LOWERBODY][CATEGORY:BODY_LOWER]
[DEFAULT_RELSIZE:1000]
[BP:NK:neck:STP][CON:UB][CATEGORY:NECK]
[DEFAULT_RELSIZE:150]
[BP:HD:head:STP][CON:NK][HEAD][CATEGORY:HEAD]
[DEFAULT_RELSIZE:300]
[BP:RUL:right upper leg:STP][CON:LB][LIMB][RIGHT][CATEGORY:LEG_UPPER]
[DEFAULT_RELSIZE:500]
[BP:LUL:left upper leg:STP][CON:LB][LIMB][LEFT][CATEGORY:LEG_UPPER]
[DEFAULT_RELSIZE:500]
[BP:RLL:right lower leg:STP][CON:RUL][LIMB][RIGHT][CATEGORY:LEG_LOWER]
[DEFAULT_RELSIZE:400]
[BP:LLL:left lower leg:STP][CON:LUL][LIMB][LEFT][CATEGORY:LEG_LOWER]
[DEFAULT_RELSIZE:400]
[BP:RF:right foot:right feet][CON:RLL][STANCE][RIGHT][CATEGORY:FOOT] [GRASP]
[DEFAULT_RELSIZE:120]
[BP:LF:left foot:left feet][CON:LLL][STANCE][LEFT][CATEGORY:FOOT] [GRASP]
[DEFAULT_RELSIZE:120]
It's just a copypasta of HUMANOID_ARMLESS_NECK, which all birds use.
Second, go to your graspybirds (creature_whatever.txt) and change [BODY:HUMANOID_ARMLESS_NECK:BLAH:BLAH] to the new one you made, HUMANOID_ARMLESS_GRASPTALONS. You can name it whatever you want as long as it's unique.
And you're done!
Ninjaedit: I say 'need' to make a new body but there's actually a few different ways to go about it. This is probably the least painful though.