Birds. Birds have their whole economy modified to subserve their great functional peculiarity of flight.
Immense muscles are required for the downward stroke of the wing, and to give attachment to these the sternum has a strongly developed keel. To bring the centre of gravity low, even the muscles which raise the wing are attached to the sternum, or breastbone, instead of to the dorsal region, as might be expected; and to brace the wings back a strong furculum—the merry-thought—is attached. The breast, then, is the seat of the greatest functional activity in birds, and, consequently, we find in a vast number of birds that the breast is the seat of vivid colour.
As many birds are modified for protective purposes, the brightest species were selected to test our views, namely, the Birds of Paradise (Paradisea), Humming Birds (Trochilidæ), and Sun Birds (Nectarinidæ). In these birds it is clear that colour has had full sway, untramelled by any necessity for modification.
Nothing is more striking than the mapping out of the surface of these birds into regions of colour, and these regions are always bounded by structural lines.
Take, for instance, Paradisea regia. In this bird we find the following regions mapped in colour:—
| Sternum | brown. |
| Clavicle | yellow. |
| Pelvis | yellow. |
| Band | brown. |
| Frontal bone | black. |
| Parietal bones | green. |
| Occiput | yellow. |
A beautiful ruff emphasizes the pectoral muscles, and the tail appendages emphasize the share-like caudal vertebræ.
If we turn to the other species of this genus, we find in P. Papuana the claret breast suddenly change to green at the furculum; and similar changes take place in P. speciosa, while in P. Wallacei and Wilsoni this region is decorated with a wonderful apron of metallic green.
The region of the furculum is equally well marked in the Toucans and Sun-birds.
If now we observe the back of a bird, and view the skeleton with the wings at rest, we shall find it falls into three morphological tracts. First, the shoulder, or scapular track; second, the thigh, or pelvic; third, the tail, or caudal region; and in all these birds the several tracts are beautifully marked by sudden and contrasted change of colour. In P. Wilsoni all the tracts are brilliant red, but they are separated by jet-black borders. In Nectarinea chloropygia the scapular region is red, the pelvic yellow, and the caudal green.