Figure 2. Experimental setup and phenotypic classification.

A) Melanin-based plumage pigmentation differs between of all-black carrion crows (CC) and grey-coated hooded crows (HC). Growing feather follicles were sampled in the head region (circle) and on the ventral part of the torso (rectangle). Photographs adjacent to the schematic representation of birds show an example of semiplume, pennaceous body feather (bar = 1 cm) from torso or head for each taxon. Squares define the area of the mature feather represented by the ensheathed feather follicle (bar = 2 mm) which forms the raw material of the experiment. Lighter pigmentation of mature feathers of HC torso is already visible in ensheathed feather follicles. Symbol color imitates the pigmentation of mature feather tips. [Bird drawings courtesy of Dan Zetterström].

B) Bright-field images from sections of ensheathed feather follicles. The dashed square on the longitudinal sections defines the corresponding position of the cross section at 1000 μm above the dermal papilla. The arrowhead in blue indicates the location of the rachis (see Figure 1). Dark areas represent regions where light-absorbing eumelanin is accumulated. Follicles from black-fathered regions show relatively higher eumelanin content in barb ridges than those sampled from the grey torso of hooded crows; bar = 0.5 mm.