P. W.). Figure 3a shows one reconstructed cross section of a willow warbler feather, Fig. 3b the same cross section after segmentation and Fig. 3c the cross section after editing with the two perpendicular buy Belnacasan principal axes used to calculate the second moments of area. The images sometimes showed gaps in the lateral wall of the shafts (see Fig. 3b and c). There were in fact no gaps in the shaft and this phenomenon is a result of the image reconstruction and editing process: (1) portions of the lateral walls of the shaft are in some cases so thin

that they cannot be resolved in the image processing; (2) during hand editing, it often proved difficult to determine the limit of the shaft in the regions where barbs emerge and are close to the shaft; the removal of small volumes belonging to the lateral wall of the shaft only leads to a very small underestimation of the dorso-ventral second moment of area, because these volume elements are very close to the dorso-ventral bending axis. Each stack of bitmaps representing a scanned feather shaft segment was read into a three-dimensional matrix. The volume of keratin in the scanned shell (cortex) segment was determined by counting the number

of matrix elements representing keratin in the dataset, multiplied by the voxel volume of 20.35 μm3. The dorso-ventral and lateral bending axes were determined as the two principal axes of an anterior–posterior GSK2118436 cell line projection of each cross section (Nash, 1977; Kranenbarg et al., 2005). The second moments of area were averaged over all images of each scan. All calculations were performed in matlab® 7.0. The general morphology of the rachis is very similar in the two species. The rachis has an approximate shape of a box girder; it consists of a compact shell (cortex) and is filled by the substantia medullaris, which contains air-filled dead cells and which is not visible in the scans. Neither transverse septa, a ventral grove nor dorsal ridges could be observed in RG7420 supplier the scanned segments (for a comparison with pigeon primary feathers, see Purslow & Vincent, 1978); septa can, however, be seen in the more proximal parts

of the shaft (data not shown). Both lateral portions of the cortex from which the feather vane projects, are very thin. The central portion of the dorsal region and both ventral corners of the rachis are reinforced. The rachis is mainly designed to withstand dorso-ventral bending; generally, the second moment of area with respect to the dorso-ventral axis is roughly twice as big as the values with respect to the lateral axis (we only report values of I with respect to dorso-ventral bending). There is a strong positive relationship between cortex volume and the second moment of area I; this applies to the pooled data (r=0.88, n=42, P<0.0001) and for each single species (willow warbler: r=0.82, n=23, P<0.0001; chiffchaff: r=0.92, n=19, P<0.0001).