COURTSHIP ACTIVITIES IN THE RED-THROATED DIVEU. 277 
remains passive, to express it physically by the natural channel proper to 
the instinct which it accompanies. What will the consequence be ? Tt will 
be that the psycho-physical energy will discharge into other motor channels, 
since it is debarred from discharging into its proper and normal channels. 
Perhaps the most frequent way in which it is discharged in birds is by 
rapid, restless movement, accompanied by droojjing and partial spreading of 
the wings, spi-eading of the tail, and ei-ection of feathers, especially on the head 
and breast. This type of action seems to have had a dual origin. In the first 
place^ it is often very similar to the actions normally adopted at coition, e. g., as 
Mr. Howard informs me, in the Buntings. In the Warblers the similarity is 
still marked, but to a lesser degree. As I have suggested, it is probably the 
direct result of the nnsatisfied desire for coition. In the second place, 
however, it appears to represent the result of a general state of excitement, 
quite apart from any representation of coition attitudes. It is often difficult 
in any particular courtship ceremony to disentangle the share of the two 
contributions. In the Grebe, for instance, the ordinary ceremonies are in no 
way representations of coition actions, while in the Buntings they are almost 
■wholly so. In any event, the dual origin of the ceremonies as a whole is 
clear — general expression of hyper-excitation combined with more or less of 
the typical coition attitudes. 
Once this type of action exists, however — and it will inevitably come into 
being in some form or other in the conditions which we have outlined, — it 
will naturally form the chief basis for the development of specialized court- 
ship actions and courtship colours and structures belonging to the stimulative 
type. When it is the wings and tail which are spread and the crown and 
breast-feathers which are erected, it will be inevitable that, if further stimu- 
lation is advantageous, it will in the main be achieved by making these parts 
more conspicuous (so as to accentuate the difference between the normal 
appearance of the bird and its appearance under the. influence of sexual 
emotion) ; and this will be most readily done by the development of bright 
colours on these parts, or by the formation of special structures in these 
regions — structures which are usually displayed in full only under the 
influence of sexual emotion. Since the form of the wings cannot be much 
altered without interfering with the efficiency of flight, it follows that special 
courtship structures will not often be developed in connection with them ; 
and as a matter of fact we find that the chief special courtship structures 
are crests on the crown (as in Peacock, Herons, Goldcrest, Peewit, Cranes, 
&c.) or breast-plumes or other structures on the breast (as in Herons and 
Egrets, Bustard, Umbrella-bird, &c.), or elongations or other specializations 
of the tail {e. g. Pheasants, Humming-birds, Blackcock, Lyre-birds, &c.) 
or of the tail- region (Peacock, &c.). 
The erection of feathers on other parts than crown or breast is also common, 
and its specialization gives us structures like the ruff of the Crested Grebe 
and that of the Rufi', the elongated scapulars of Herons, and those, both 
