254 MR. JULIAN S. HUXLEY ON 
5, In many birds similar in the two sexes both male and female incubate. 
In many but not all such species the relieving of one bird by the other on 
the nost is the occasion for a special sexual ceremony, which in its turn may 
or may not be used at other times. This ceremony of nest-relief appears to 
be absent in the Grebe. 
With this brief introduction, we may pass to the observations on the Red- 
throated Diver. These were made while the writer was on the Oxford 
University Expedition to Spitsbergen, 1921. Most were made on Prince 
Charles Foreland, an island 60 miles long, off the west coast, on which a 
small party of us camped for 11 days, from June 30 to July 10. Numerous 
small freshwater pools are found here, as in many other places near the coast 
in Spitsbergen ; and on a great many of these, at any rate on the north- 
western and western coasts, Divers are to be found nesting. We were 
fortunate in being encamped on the shores of Richard Lagoon, a sheet of 
brackish water about a mile wide and 5 miles long, communicating with 
the sea by one narrow opening. This lagoon was the scene of much of the 
Divers' courtship, both pre- and post-mating, though especially the former. 
Much of the birds' feeding was done on the open sea : but for rest and court- 
ship the lagoon was chosen. Even after incubation had begun, the birds 
appeared to come down to the lagoon when not sitting ; it was the scene of 
veritable social gatherings of Divers, four or five being commonly on its waters 
together, and eight having been seen there on one occasion. Sometimes they 
would fish, progressing by means of long dives, as does the Crested Grrebe ; 
at other times they would stay quiet, resting or preening themselves. But 
during the period of our stay we never observed two or more Divers on the 
lagoon without some courtship action following within a comparatively short 
time. The male and female are similar in appearance ; the female is smaller 
than the male, although the wing and beak measurements of species overlap 
somewhat. However, the female appears to have a much less massive neck 
than the male, and a separation of the sexes can usually be made with 
reasonable probability. 
Description of Chief Courtship Actions. 
It will be best to begin with a description of the chief types of actions 
seen, following this with their probable interpretation. 
1. " Plesiosaur race." This ceremony was so christened because the 
attitude adopted by the birds in its performance made them resemble miniature 
Plesiosaurs half out of water (fig. 1). Usually two or three birds took part 
in it, sometimes four. The birds depressed the hinder half of the body below 
the water ; the body was held at an angle so that the breast and shoulders 
were stuck out ; the neck was stuck upwards and forwards in a stiff position, 
the head and ^eak inclined somewhat forward, again somewhat stifHy. In 
this attitude the birds swam through the water, accompanying one another. 
