To measure the maximal bite force we used a force
transducer (Aikoh, Osaka, Japan; 9000 series) mounted with
two flat metal plates (Fig.·1). The birds were held by hand and
trained to bite the metal plates. The birds only used their beak
tips to bite the force transducer and refused to bite at more
caudal positions within the beak. Bite force measurements were
performed several times in a row at each occasion, and on at
least five different days to determine the maximum bite force
at the tip of the bill. The maximal bite force for a bird is the
highest value measured, but in all cases at least two other bite
forces were recorded that differ less than 0.2·N of the maximal
value.