LIFE AXD BEHAVIOR OF THE CUCKOO 
179 
not, which was introduced into its nest. (See p. 224) . In a general 
way it has been found that in certain famihes and species of birds 
foreign eggs or other objects are more uniformly accepted than in 
others. Thus while pigeons will usually accept anything, thrushes 
are more discriminative, but the behavior of any particular thrush 
will depend upon its individual characteristics at the moment of 
introduction, and upon the intensity of the brooding impulses 
at that time. 
Leverklihn, who has tabulated all the experiments, made up to 
his time, on the behavior of birds towards foreign eggs, placed in 
their nests (although he has intentionalh- left the cuckoos out 
of his account), found that these intrusive objects were received 
and brooded about as frequentlj^ as they were destroyed, or aban- 
doned together with their own proper eggs. His extensive tables 
embrace 222 species of birds of nearly all the families, observed 
by many naturalists chiefly in Europe and America, during 
more than a century, and include 406 cases, where definitive results 
were noted. 
Although Baldamus maintains that all cuckoos' eggs, both 
native and exotic to Europe show a great similarity to the 
eggs of their nurses, we do not consider the question to be difi- 
nitely settled. ^Meantime we are inclined to regard the variabil- 
it}^ of the cuckoo's eggs as a lingering expression of a general 
variability which this bird must have undergone previous to and 
during the disturbance of its rythmical reproductive activities 
entailed by the lapse of the impulses to build a nest and tend its 
young. 
11. The cuckoo is said by Baldamus to prefer those nurses, 
in the nests of which it has itself been reared, a statement ob- 
viously difficult, and except in the rarest cases impossible to prove 
without 'banding' the bird; to lay no egg in suitable nests, if 
observed by man, or if those nests have been disturbed or touched : 
to seek to introduce its egg in the absence of the nest-owner, 
and to carry off the laid egg in bill if, in laying, it has been observed. 
It would seem that touching the nest could have no possible 
effect, unless we attribute to the cuckoo the keenness of scent of 
