A Cooperatice Studi/ 
319 
but the proportions vary with the size of the clutch ; for we have 
74 brown to 63 green eggs in the clutches of 1, 
153 brown to 203 green eggs in the" clutches of 2, 
216 brown to 393 green eggs in the clutches of 3, 
or 100 to 85, 100 to 133, 100 to 184 brown to green eggs respectively. In other 
words the proportion of green to brown eggs increases with the size of the clutch. 
Those readers who will examine Plate VIII in the first memoir* will see how 
distinct the brown and green ground colours are, and will understand how necessary 
it is to find some explanation for the change in proportions of colour as the clutch 
increases in size, and for the mixture of colours in the same nest. The field- 
workers appear to be confident that the same bird can lay different coloured eggs, 
basing their statement apparently on diversity of colour appearing in clutches of 
eggs having the same size or shape. 
The hypotheses that suggest themselves are : 
(i) That the common terns consist of two gentes one of which lays brown and 
the other green eggs. The mixture of colour arises from the existence of ' cuckoo ' 
terns who lay in other hen's nests. 
We cannot ascertain the number of brown egg-hiying tern ' cuckoos ' who lay 
in brown egg nests or of green egg-laying tern ' cuckoos ' who lay in green egg 
nests. But if the 19 BG arise from cuckoo- terns, we must originally have had 
74 -I- 63 4- 19 single egg nests and in these 156 nests 19 tern ' cuckoos ' of opposite 
colour laid. The chance therefore of a tern ' cuckoo ' of opposite egg colour laying 
in the 1 egg nests is "1218. Treating the 2 egg nest in the same manner, we 
have 67 -f- 92 -f- 8 -I- 14 = 181 of them and in 22 we have occurring the egg of the 
tern cuckoo of opposite colour, or the chance is 1215 ; this number is sub- 
stantially the same as we reached before and the coincidence is remarkable. But 
it collapses when we go a stage further. We have 62 \- 119 whole colour clutches 
of 3, we should therefore expect 25 clutches of 4 with composite colours, i.e. 
25/(62 + 119 + 25).= '122 neai'ly. Now only a single 4 clutch nest was found and 
this had all green eggs. With a chance of about 1 in 8 that a cuckoo-tern will 
lay in any nest, it is hard to believe that it missed at least 181 nests. It appears 
that three eggs is the practical limit to the size of the clutch laid by one hen, but 
it seems hard to believe that the cuckoo-tern would avoid all nests which ah-eady 
had three eggsf, i.e. the cuckoo-tern hypothesis seems to involve a considerable 
percentage of composite four egg clutches, which do not appear. This argument 
seems sufficient to render the hypothesis very improbable. 
(ii) There is only one gens of the common tern which can lay both brown 
and green eggs. Since, however, the number of green eggs increases with the 
size of the clutch, it is not possible to consider the chance of laying a brown, 
* Biometrika, Vol. x. p. 140. 
t Or that the rightful owner having laid two eggs would refrain from laying the third because the 
'cuekoo' tern had already laid it. 
