In both the fringillids and the estrildids a seed is picked up
and transported to the back of the beak and placed between the
rims of the beak. This often requires only limited number
(approx. 3) of beak movements. Once the seed is placed
correctly between the rims of the beak a cracking attempt is
made. This can be recognised by depression of the elevated
upper jaw onto the lower jaw. If the cracking attempt is successful part of the split husk becomes visible at the outside
of the beak. If the cracking attempt was not successful the seed
is repositioned between the rims and another attempt is made.
The preliminary analysis of the cracking technique shows that
the lower jaw makes a lateral movement just before a cracking
attempt. During this movement the tip of the lower jaw moves
in a direction opposite to the side where the seed is cracked. Its
amplitude is much smaller in the Java sparrow than in the
greenfinch. In the Java sparrow the movement is about 1·mm,
whereas the movement of the lower jaw of the greenfinch is up
to 4·mm (Fig.·5). Note that in both cases the amplitude of the
movement is clearly larger than for the rigid upper beak
(measurement error).