known with, an accuracy of t t day. It ws# about lot |«|i for 
the first mn and e eb«de longer for th second. The difference© 
for th» two 'eggs la probably eigatfioent and met reflect the 
relative Inefficiency of the first day or two of incubation, ao 
the egg* are usually laid at an interval of two days anti 
incubation etarte with the first* 
If <* 
•% ' ' , • * 
The knowledge Of flag date • in the different nests 
w&s need for son# thing wise, ia well known that the terns 
will ewe at and even MfcJlSit ruder* who w about in the 
oniony # especially when the sgga are hatching. 3y offering 
a standard "predator stimulus* and by scanting the number of 
swoopa node by the brooding bird of a. particular neat I could 
©essur* fluctuations In the tern© aggressiveness towards 
predators* in this woe on# could aee how far the behaviour 
oerrobor* ted exiwotstion* An eight be expected it w«. neess&ary 
to 'be sur® of the identity of the brooded a» the two ;>&n ■& often 
reacted qutt“< differently* Thor© was no indication that in all 
pal re one sex swooped more than the other. The variation in 
response of one bird between on© day and the next was rather 
slight which suggested that it was not subject to seat* short 
tern fluctuations impending,, for s&aeplt* on whether the bird 
had Jus t taken over brooding fros its ©ate* This was Just as well 
because only the minimum an uat of disturbance w&a possible. 
In fact there were no desertions a© a result of the experiment*. 
T he behaviour of *> , ; mir# was studied and in e&eh 1 could 
tell which bird wm sale and which female. The completeness of 
the record of at tanks varied fro© .-air to pair and it was clear 
that there was a very great variability between different pairs. 
Some os ire never sits eked at all* in others only one of the 
oars ni s ever st tacked* But if this variation it ignored 
Mid r- * 1 the data ore pat together it is poassible to sal sale to 
the average xstober of attacks for each of the Vday periods 
fro© the date the first egg was laid* I havs used only the 
dnt" from period* Day A to Day $0 because outeiSe this span 
all the observations were based at a single neat. The following 
table shows the average number of attacks in each of the r >«day 
periods. . . '* 
Day 
6*10 
f |*lg 
2 i -.1g 
2IW $0 
Total no, of 
It 
dl 
IT 
>ia4 
og 
swoops 
Ko* of test® 
1 1 
«g 
16 
31 
fiean no, of 
swoops per test 
1.36 
p , or 
*.31 
r * . 46 
1,06 
Thus th© expected increase In attacks was found in that ., eriod 
when the young hatched. There also appears to be an eoturl 
increase fro® tine beginning of Incubetion up to the tl«o of 
hatching* This sight have occurred by eh&noo an’ it is necessary 
to © iy static 1 tests to find if this trend is not ueroly duo 
A 
