Fringillids and estrildids differ in their husking time on hemp
seeds, independent of maximal bite force. Fringillids are on
average faster than estrildids, especially when their bite force
is low (statistically when the bite force is less than 7·N; see
Fig.·3). Frame by frame analysis of a limited set of video
recordings of the seed cracking process in finches and estrildids
showed that husking time comprises two different phases (van
der Meij et al., 2004). During the transport phase, the seed is
transported to the back of the beak and positioned next to
its rims. The number of beak movements (mandibulations)
required to transport a seed is the same in finches and estrildids,
and seems largely independent of seed size. During the next
phase the seed is manipulated to position it between the rims
of the beak. This often requires a number of small amplitude
opening and closing movements of the beak. Once the seed is
positioned correctly, a cracking attempt is made. If the cracking
attempt fails, the whole process is repeated until the cracking
attempt is successful. The number of positioning movements
per cracking attempt increases with seed size, reflecting the
difficulty in manipulating large seeds. The number of cracking
attempts clearly increases with seed hardness and the largest
contribution to differences in husking time comes from the
number of cracking attempts (Van der Meij et al., 2004). In
birds that use a long time to crack a seed, many cracking
attempts fail and the seed is often squeezed from between the
rims of the beak into the oral cavity. The difference in husking
time between estrildids and fringillids may therefore be related
a difference in the way the two groups handle the seed during
cracking attempts.