1 66 SEA-BIRDS 



selection; it was Selous (1905, 1905-06) who pioneered the general 

 concept of unilateral versus mutual display, to be followed by Huxley 

 (1914) who, with his papers on the striking courtship of the great 

 crested grebe, introduced its importance to professional biology. 



The mutual ceremonies of the sea-birds are hardly less remarkable 

 than those of the grebes. They are prolonged throughout the summer, 

 long after mating has taken place and the young have hatched, but 

 their function is a useful one: mutual display serv^es as an emotional 

 bond to keep the monogamous pair united so long as parental cares 

 continue. This interesting biological device is of course of no value 

 to the polygamous land-birds, in which the male takes absolutely 

 no interest in the family. It is evident, then, that the relation of the 

 sexes to the young and to each other determines the type of courtship 

 display as well as the degree of development of the external sexual 

 characters (colour and plumage) in male and female. Sexually faithful 

 husbands resemble their wives so closely as a rule that the observer 

 often cannot distinguish between the sexes of sea-birds in the field — 

 for even the act of coition is sometimes reversed. Mutual display — 

 leading to mutual stimulation — is advantageous where there is such 

 outward uniformity. 



It is interesting to trace the origins of sexual display. In courtship 

 all sea-birds make use of the bill in rubbing, fencing, scissorsing, 

 fondling or preening motions; in doing so they are unconsciously 

 copying the familiar movements of feeding and being fed as chicks. 

 Gaping is a characteristic of some sea-bird courtship ; it too is derived 

 from gaping when being fed in the nest. Threat attitudes, involving 

 the use of the bill, the raising of neck-feathers, and often wing-play, 

 can be traced to the behaviour of the growing chick when it first 

 defends itself from its nest-mates (as when securing food at feeding- 

 time) or other rivals or enemies; these threat attitudes become incorp- 

 orated in, or adapted to, the courtship display. 



How early in its existence the young sea-bird receives the first 

 mdelible impressions of the voice and behaviour, including the mutual 

 display, of its parents we do not know — probably the precocious gull, 

 tern and skua chick does so as soon as it hatches. Lorenz (1935) has 

 shown that by taking the place and duties of the parent wild goose at 

 the moment of hatching he transferred to himself the allegiance of the 

 gosling, including its responses of sexual behaviour which develop at 

 the appropriate age. This imprinting of parental influence is the 

 first step in the socialising of the chick, which proceeds rapidly during 



