6 
But the males hav© soar® than one way of attracting a female 
Into their vicinity* Such birds, ©specially if they have not 
been vial tad ®ueh by females* my perform an advertising flight. 
Thi Is mad# to and fro over the colony area bat sob© ti =. 
t& v «8 a male farther afield* In this flight ho sake* hi ns self 
thoroughly conspicuous fifing rather slowly and calling 
frequently. Ho is inclined to approach other flying birds as 
well as birds on the ground and he Mens to try and induce 
the other bird to fly up and Join with hi® In safe ruing a 
ceremony .1 have called the Pass. In this on© bird files past 
and over the other and takes up a position in front of It. 
As it psat#»> the overtaking bird assume# a posture which seems to 
be the same as the ground stretch posture, so far , as this is 
compatible with flight* and this posture Is maintained as he 
files In front of the other, the bird who is overtaken 
adopt® what seems to be the aerial equivalent of the bent. An 
unmated ©ale unking the advertising flight tends to adopt the 
aerial bent posture at the approach of or flying near of another 
bird and he Is inclined to fly toward* the other bird, this 
seems to induce the other to perform the stretch role in the 
pass. The flying- over bird not only has to adopt the stretch 
posture but he also has to follow the manoeuvres of the bent bitbU 
That this is don# from a position in front of the bird to bo 
followed sees® rather surprising but there la m doubt that 
the terns are well able to do this. Indeed if the manoeuvres 
of the bent bird, are sc sharp that the stretch bird falls out of 
position and cones behind the other he again overtakes and comes 
to fly In front. 
In tense ' its pur >*• the ceremony of the pass seems 
to test how far the stretch bird 1® content to follow the 
lead of the other, This Is supported by the later behaviour 
of the bent bird who, when he has been successful in finding 
a female who will stretch over hi© and follow hid movements, 
will lead her to land on his territory. As he lmnds and she cones 
down beside hi® both enact the normal greeting on the ground 
which was described above* 
The behaviour filth which a malm on the ground attract* 
and greets a female fens certain important features in common with 
he advertising display flight, neither occurs i very aggressive 
males* Both occur or become fully expressed as a reaction to 
other birds nearby, especially when those birds show sose 
Inclination to approach the male in the at ret eh pee bare but not 
when the ©tier she wo hostility. Finally the purpose of both 
displays seetfjs to be to focus the* interest of the female - either 
to draw her attention fro® the colony area as a whole to a 
particular territory or from the territory as a whole to the iv lg*& 
scrape* The latter Is important because the scrape or scrapes 
which he uses sre not necessarily in the raid He of the arem which 
he defend®. It would presumably lead to the disadvantages of 
prolonged fighting if the female laid in the scrape of a neighbour 
