266 MR. JULrAN S. HUXLEY ON 
and especially the relations of the sexes. Those who have opportunity, 
patience, and a good glass, and are willing to take full notes, will find that 
steady observation of almost any species of bird at the beginning of the 
breeding-season, particularly if a single pair can be followed throughout, 
will bring results which may be of ver^^ considerable interest, both from the 
standpoint of pure biology and also from that of comparative psychology, as 
well as being in itself a very fascinating occupation. 
The following paragraphs must therefore be taken only as a provisional 
interpretation of the observations which have so far been recorded. 
The Red-throated Diver is monogamous ; possibly it pairs for life. If it 
pairs for life, it follows that the pairs will have some form of association even 
through the winter. In any case, however, a change occurs in spring. 
Either the loose association of male and female is replaced by a much more 
intimate onej or else the birds have been really unmated during the winter, 
and now do separate out into pairs. This must, of course, be the case with 
birds which have not previously mated. In any event, the change is definite, 
and warrants the term "pairing-up" which is usually given to it. This 
change is presumably associated with a change in the reproductive organs, 
which start to secrete their specific hormones. 
The pairing-up appears, at least with some individuals, to occur in the 
winter haunts. When so, it is followed, probably shortly, by migration, in 
pairs, to the breeding-grounds, where each pair as soon as possible stakes out 
a territory for itself. The present species appears to breed on small pieces 
of water, and probably the territory is, in most instances, coterminous with 
the nesting-pool. 
The fact that the pair migrate together and choose the nest-site and 
territory together is in strong contrast to the occurrences in such Passeres 
that have been fully investigated, such as the Sylviidse and the Emberizinse, 
in which the males alone stake out territory (Howard, '20), and, if migration 
occurs, migrate before the females. However, it seems to occur in many 
Pygopodes (Bent, op. vit.) ; and in Herons (unpublished observations of my 
own), although migration is in flocks, the choosing of the nest-site is under- 
taken by the pair together. 
Although from the evidence of others it appears certain that pairing-up 
may occur before migration, yet my own observations appear to point to this 
not being universal. The fact that the " Plesiosaur " ceremony occurred 
frequently during the first few days of our stay, but later was absent or very 
rare, as well as the fact that the simplest interpretation of it was that it was 
concerned with the "choosing" of mates, indicates that probably a good 
many birds arrive unmated on the breeding-grounds. Possibly (I throw this 
out merely as a suggestion) the birds which have nested before, pair up in 
winter quarters, while those which are about to nest for the first time do not ' 
do so until they arrive on the breeding-grounds. 
