To study the cracking technique high-speed video recordings
(color high-speed video; S-VHS; Nac, Beringen, Belgium,
250·fr·s–1) of the Java sparrow (estrildid) and the greenfinch
(fringillid) were made. Up to 16 markers were placed on both
sides of the bill and on top of the head. The birds were offered
hemp seeds on a small plateau surrounded by three mirrors
(left, right and overhead; Fig.·2) at an angle of 45° to the frontal
plane. The co-ordinates of markers on the head and of markers
visible in the mirrors were digitised and the three-dimensional
(3D) position of the markers was reconstructed using the direct
linear transformation technique (DLT) (Woltring and Huiskes,
1990). The DLT transformation was based on a 3D-calibration
object with 15 spherical markers.