Interrelation among descriptive contour feather variables.

Variables describing contour feather structure from the sternal tract of great tits. Total feather length (without calamus); densities of pennaceous (Pn) and plumulaceous (Pl) barbs and barbules; and proportion of each feather composed by plumulaceous barbs. Eigenvalue and coefficient of determination of the first factor obtained from a principal component analysis summarising overall feather structure, together with the factor loadings of each variable.