and Fringillidae. In hemp seed the husks are fused and form a
closed shell around the kernel (Van der Meij et al., 2004). In
contrast to so-called open-shelled seeds, in which the two husks
envelop the kernel only loosely, birds need to apply
considerable force on the seed before the husk splits into two
parts. In open-shelled seeds the husks can be removed very
quickly without actually cracking the seed coat (Kear, 1962).
The mean hardness of hemp seeds was 12.16±4.95·N. The
hardness of the seeds was measured by stepwise lowering of a
force transducer pressing on individual seeds, and recording the
peak force at the point where the seed husk cracked (see also
Van der Meij and Bout, 2000).