178 
FRANCIS H. HERRICK 
The commonest dupes of the cuckoo in Great Britain are the 
titlark or meadow pipit, the pied wagtail, the reed warbler, hedge 
sparrow and the robin redbreast (Erithacus rubecula) , in some of 
which as in the case of the hedge sparrow the introduced egg is 
frequently very unlike its fellows. If we assume with Newton^" 
that the eggs of certain cuckoo gens have come to resemble those 
of certain nurse-birds whose nests they habitually steal, through 
the cumulative effects of inheritance and selection, upon the 
basis of the advantage of escaping detection thus secured we 
meet with fresh difficulties, for it is further assumed that birds 
may be divided into two classes, — those which are readily duped 
and those which are not, but such a classification will not hold. 
Expressed in another way this might mean that the parental 
impulses are stronger at a certain time, as when the eggs are laid, 
in certain species than in others, and this is true. The question 
involves, however, too many variables, one of which is fear, and 
the degree of fear or timidity, being determined in a large meas- 
ure by experience varies, at different times in the same indi- 
vidual. Many birds, moreover, which would ordinarily belong 
to the ^wise' or 'timid' class, suddenly become 'stupid' or 
'bold' with the rise of the brooding instinct, and the <;orres- 
ponding suppression of fear. This is particularly well illustrated 
in cedarbirds, rose-breasted grosbeaks, and even in the yellow- 
billed cuckoos, upon all of which the cowbird has been known to 
inflict its eggs, though in every case very unlike their nest- 
mates. 
The whole subject of the effect of foreign eggs, placed in other 
birds' nests, by birds of the same or different species or by man,, 
has been thoroughly investigated by Lever kuhn,ii and Watson^^ 
has shown how the disposition of the noddy tern is suddenly 
transformed by the sight and touch of an egg, whether artificial or 
Newton, Alfred, and Gadow, Hans. A dictionary of birds, p. 124. London, 
1893-1896. 
Leverkiihn, Paul. Fremde Eier im Nest. Ein Beitrag zur Biologic der Vogel. 
pp. i-xii, 1-212. Berlin, 1891. 
" Watson, John B. The behavior of the noddy and sooty terns. Pub. Car- 
" negie Institution of Washington, pp. 103, Washington, 1909. 
