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First published online August 17, 2006
Journal of Experimental Biology 209, 3329-3335 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02379
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Seed husking time and maximal bite force in finches

M. A. A. van der Meij* and R. G. Bout

Department of Evolutionary Morphology, Institute of Biology Leiden, PO Box 9516, 2300 RA Leiden, The Netherlands


Figure 1
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  		Fig. 1. Tool for bite force measurements. Rigid metal plates (A) that are slightly pressed together by the bill as a bird bites on the plates (notice the rounded ridge to prevent pressure of the rest of the bill). B, connection to the force transducer.

 

Figure 2
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  		Fig. 2. Image of Java sparrow during seed cracking in experimental setup. The head is recorded directly and in three mirrors: frontal (A, direct view), left lateral (B), dorsal (C) and right lateral (D).

 

Figure 3
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  		Fig. 3. (A) Exponential decrease in husking time with increasing bite force for estrildids (grey squares) and fringillids (black triangles). The data are fitted with an S-curve for each family. (B) Ln-transformed husking time vs inverse bite force for estrildids (grey squares) and fringillids (black triangles), to show 95% confidence intervals.

 

Figure 4
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  		Fig. 4. Cumulative hazard function at the mean of bite force for estrildids (grey squares) and fringillids (black triangles).

 

Figure 5
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  		Fig. 5. Movement of the upper (grey) and lower jaw (black) in the greenfinch (A,B) and Java sparrow (C,D) during a cracking attempt. A and C show the lateral movement (in mm) vs time (frames). While the lower jaw moves sideways during opening of the beak, the tip of the upper jaw does not move in the horizontal plane. Arrows indicate positions with closed beaks (opposing beak tips). B and D show graphically the most extreme positions of the lower jaws in the horizontal plane. The beak tips are pointing upward. On the x axis, zero coincides with the medial plane. On the y axis the position of zero is arbitrary.

 

Figure 6
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  		Fig. 6. Schematic representation of a cross section through the upper and lower beak of the Java sparrow. (A) In the resting position the lower jaw would press just off centre onto a 2.5 mm seed, generating a force along the surface of the seed. (B) Biting force might be directed more efficiently perpendicular to the surface of the seed by moving the lower jaw in a direction opposite to the side where the seed is cracked (large arrow in A).

 




© The Company of Biologists Ltd 2006