Xbows don't seem to get headshot damage, but I knocked one out using a regular bow and was getting headshot damage everywhere but the head. Knocked out a 135 with 15 arrows, where a 130 took 40 with an xbow. Both were 100% damage.
An argentavis can't fit through a ceiling with one tile missing, but a pteranadon or lymantria can. Build a trap with one corner ceiling missing, aggro the argentavis, fly in one side and out the missing ceiling tile, and land. Dismount, run around, and close the entry door. Tame safely at your leisure.