The difference in amplitude of lateral lower jaw movement
between estrildids and fringillids may be related to a difference
in preferred seed type. Estrildids are generally believed to feed
mainly on open shelled seeds, and remove the husks without
actually cracking the seed coat (Kear, 1962). Although the
force with which open-shelled seeds are dehusked cannot be
measured directly it is very probably low. Husking times for
open-shelled seeds are low in comparison to husking times for
closed shelled seeds and very similar in estrildids and
fringillids of various sizes (R.G.B., personal observation).
Fringillids, on the other hand, feed primarily on closed-shelled
seeds that need to be cracked. As the number of failed cracking
attempts may be expected to be lower when biting forces are
low, small amplitude lateral movements are not necessary for
species that feed mainly on open-shelled seeds.