The flexural stiffness of the rachis varies along the length of a prim
ary feather, between primaries and between species; the possible contr
ibution of variations in the longitudinal Young's modulus of feather k
eratin to this was assessed. Tensile tests on compact keratin from eig
ht species of birds belonging to different orders showed similar modul
i (mean E=2.50 GPa) in all species apart from the grey heron (E=1.78 G
Pa). No significant differences were seen in the modulus of keratin fr
om primaries 7-10 in any species. There was a systematic increase in t
he modulus distally along the length of the rachis from swan primary f
eathers. Dynamic bending tests on swan primary feather rachises also s
howed that the longitudinal elastic modulus increases with increasing
frequency of bending over the range 0.1-10 Hz and decreases monotonica
lly with increasing temperature over the range -50 to +50 degrees C. T
he position-, frequency- and temperature-dependent variations in the m
odulus are, however, relatively small, It is concluded that, in the sp
ecies studied, the flexural stiffness of the whole rachis is principal
ly controlled by its cross-sectional morphology rather than by the mat
erial properties of the keratin.