COURTSHIP ACTIVITIJES IN THE EED-THEOATED DIVER. 275 
territory and the need for nesting at the proper time for supplying the food- 
requirements of the young. 
Such species, where territory is of major not of merely minor importance 
[feeding territory as opposed to merely nesting territory), we may for con- 
venience call " territory birds." In them, during the female's second phase, 
there is comparatively little "courtship''' on the part of the male. When 
the female feels impelled to coition, she. adopts a particular attitude, and 
the male immediately performs the act. The bulk of the "courtship" 
(excluding song) is due to the fact of the existence of the "engagement" 
period when the male and female are in different states of endocrine exci- 
tation, and this is a biological " accident " arising as a by-product of other 
needs and functions. 
The song, on the other hand, is, in territory birds (where it reaches by far 
its highest development), connected primarily with territory — with attracting 
females to, and warning other males oft', occupied areas. The displays and 
pursuit flights are merely an expression of the desire of the male for coition, 
a desire which cannot be satisfied in the engagement period. The display is 
in origin an adoption of the coition attitude itself, or something closely 
resembling it, brought about naturally as a result of brain-structure when a 
certain level of sexual excitement is reached ; the pursuit flight is in origin 
an attempt to force the female to submit to coition. 
Once, however, these actions had arisen in this way, they formed a possible 
basis for further evolution. The fact that in large numbers of species of this 
type the male is brightly coloured, and brightly coloured in such a way as to 
display the bright colours in the above-mentioned type of ceremony, is 
evidence, though of a mei'ely circumstantial kind, that something of the sort 
has occurred, and that the ceremonies, originall}- mere attempts at coition, 
have later acquired a second and presumably stimulative function. The fact 
that the males of some territory birds are not brighter than the females 
{e.g. the Warblers) may well be accounted for on the supposition that in 
these species protection is desirable for both sexes. 
There is, further, interesting evidence to show that these ceremonies 
probably do possess some stimulative function. 
I am informed by Mr. Howard — and the statement has been in general 
corroborated to' me by Mr. Jourdain and other field ornithologists — that in 
seasons when the weather has been very cold and unpropitious during the time 
of egg-laying and immediately before, the average number of eggs in a clutch 
is smaller than usual, and that the proportion of infertile eggs is higher than 
usual. This latter effect may in part be due to actual damage to fertilized 
eggs soon after laying, caused by the inclement conditions ; it may also be 
due to failure of fertilization. The smaller number of eggs in a clutch must 
be due to failures in ovulation. It is further well known that ovulation in 
birds is, in part at least, under nervous control; the caressing of a female 
virgin dove on the back of the neck with the hand causes ovulation. 
