understand the wore la sella to reason why a bird should act 
as It does It is helpful th thin!? of the bird, as be i ng all the 
time under the Influence of one or sore of a number of tendencies 
or drives. When a bird stops .bathing and starts to r©«m one can 
say that there has been a change In the predefine ting tendency 
from bathing to oreenlng, lost of the tlae a bir^ items to do 
one thin* at a time but the fact thct towards the end of bathing 
sons birds begin to intersperse a few preening movements with 
their bathing shows 'that the new tendency can begin to express 
itself before the whole pattern of behaviour has switched over. 
This idea of fluctuating tendencies has been specially valuable 
in the study of reproductive behaviour In all cuede^ where it 
has been attempted. In the formation of the pair the mates of 
many species react to one another by attempt* to copulate, to 
file ot to a t tacit . One can, with practice , recognise all throe 
kinds of attempts and on© sees that when, the behaviour of the 
pair does not lead directly to one of the three there are 
indications that one or tiro or all three tendencies arc active. 
Indeed by using these indications one can go far to examine 
examine what underlies the indecisive-looking actions which comprise 
what we call courtship. It is useful therefore to so® how far 
it Is possible to explain the various courtship and threat 
displays la terms of these three tendencies. 
* It S I mm I I 
In my study of the Arctic tern I am still very much In 
the descriptive stage in understanding the j air formation. 
Something of the? function is already apparent but I can ohly 
guess at the causation. One© the pair bond ie well established 
the activation underlying the later behaviour is rather easier 
to tenders tend • The male starts to feed the fetalis regularly 
and cesses to show the hesitation of the early days. As he 
approaches with food she is such more ready to adopt toe typical 
Hunched Posture with neck withdrawn of a food* boiling female 
{or chick) and less likely to make the stretch as aha did In the 
early day®. The female now ©pends more tin® than before sitting 
about in the territory and she food—bege very strongly as tit© 
©al® comes in with a fish* She distinguishes hi# from other 
males with fish and seems first to recognise him whan he is 
several yards away. 
Pros the time when frequent feeding of the female begins, 
and probably often from © much earlier stag© in pair formation, 
the male show® an inclination to try and mount the female • At first 
she is usually too frightened to allow such close contact but I 
have seen a ©ale succeed In climbing on at what seemed to o© the 
beginning of the pair's acquaintance though I could never be sure 
how much they might have been together away from the territory. 
In his pre- copula tory behaviour • the male first starts to make 
© sort of parade la circles round and round the female and she,, 
at first usually trie® to moves away from him. If n movement of 
hers frightens him he will move farther from her but continue 
circling, while as he prepares to mount he get® closer and closer. 
Ac mounting becomes more Imxstnent still hie circling changes to 
walking to and fro 
