BRITISH WARBLERS 
by carrying being equally as effective, and, in fact, more often 
resorted to. Nevertheless, no one could watch the vigorous 
efforts of a bird vainly endeavouring to swallow a leaf too 
large to pass down its gullet without concluding that it was 
not due to any desire on its part to do so, but that its whole 
behaviour was congenital, and that the instinctive procedure 
being thus interrupted accounted for the extreme bewilderment 
its attitude denoted, for you may see it arrive at the nest, 
deliver the food to the young, pick up a leaf and attempt to 
swallow it, then drop it and yet again pick it up, finally after 
a pause flying away; in a few moments it returns again with 
food, delivers it to the young, and again goes through the 
same performance with the leaf. 
In whatever way we may regard such an episode, the truth 
of which anyone can test for themselves, it cannot be denied 
that the fact of a member of an insectivorous species attempt- 
ing to swallow a bramble leaf, and that by no means a small 
one, is significant. 
From yet another point of view the experiment with the 
leaf is interesting. It shows—at least I am inclined to believe 
so—that the bird has no knowledge of the relation between 
the means employed and the end attained; and it is just such 
an incident as this that increases my doubt as to whether 
intelligent modification has been that factor in the evolution 
of instinct which it is so often claimed to have been, for if 
such a combination of activities as we are here discussing had 
arisen by this means, assuredly the intelligence ought not 
to be of so low a degree as would be incapable of distinguishing 
between a leaf and a membraneous sac of excrement. 
I once witnessed a scene with a pair of these birds which 
was interesting. Finding one evening a nest, in which the 
young were just hatched, placed in such a position that it 
could be watched with little difficulty, I determined to remove 
some of the vegetation surrounding it at once, so that the 
parents might have ample time to become accustomed to the 
change by the following morning. This I did, and returned 
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