COURTSHIP OF THE GREAT CRESTED GREBE. 517 
art in many apparently very different actions of birds. Herring- 
Gulls sit or stand close beside each other for hours together, 
occasionally rousing themselves to a joint ceremony of shaking 
their necks. As the Snipe drums overhead, there is often a call 
fromthe marsh below. Many birds when paired are always calling 
to each other, and probably singing birds sing partly to their 
mate; Dabchicks have a special spring note, usually given as a 
duet. As a very simple case, I have seen a pair of Blue Tits 
very recently paired up who, although feeding, were perpetually 
ealling to each other and at frequent intervals coming close up 
side by side; it was perfectly obvious that they simply took 
pleasure in each other’s presence, like the engaged couple that 
they were. 
We have thus the following train of reasoning. Many birds 
must be kept in pairs during the breeding-season. ‘This may be 
partly effected by the instincts of the separate birds—the mstinct 
to build a nest, to sit on eggs, to feed young; and partly by 
sustincts which only can find play when the two birds are 
together. ‘These latter are often very emotional, and the court- 
ship habits of the Grebe afford a very specialized example of this 
emotional bond between members of a pair. 
1f my contention is correct, it is clear that many actions and 
structures solely used in courtship are of use to the species, and 
not only to one sex of the species; these therefore must be 
maintained by Natural as opposed to Sexual Selection, 
(iii.) Nest- building. 
I rely almost entirely on the observations of Selous (01, and 
see my Summary, section 8). 
Materials —Selous’ nest was made mainly of dark ribbony 
weed, fetched from the bottom. Some surface-weed was also 
used. Besides this, such objects as water-lily stalks and large 
water-logged sticks are occasionally employed, the latter seeming 
to help anchor the nest. To steady the nest still further, the 
weeds are often woven among the stems of growing water-plants. 
As with other birds, the materials vary with the situation. In 
Avundo-heds the nest is very largely made of bits of reed-stem, 
though always with some weed; while elsewhere no reed is used 
at all. 
Time, etc—The main bulk of the nest appears to be built very 
quickly—in a few hours, in fact. This main portion is always 
built in the early morning (as with many other birds), and while 
on one day there may be no nest to be seen, by the next it may 
be nearly completed. In this very active building both cock and 
hen often take part simultaneously; they work very hard, 
averaging between them more than two cargoes of weed every 
minute, and gomg on for half an hour or an hour at a time 
without stopping. 
After one nest has been built, another may be started (and 
