THEORIES AND CONCLUSIONS 227 
Picture a female Cuckoo, bred last season in a 
Meadow Pipit’s nest in England, returning for her 
first annual visit to the country of her birth, with 
her inherited parasitic instincts fully developed. 
What species of fosterer is likely to be most attrac- 
tive to her ? Surely that to which belongs the 
individual pair that brought her up in the previous 
season. Is it not indeed possible — even probable 
— that, for her first breeding season at any rate, 
she should not only return to the familiar surround- 
ings in which she was bred, and seek to billet her 
offspring on the species of dupe with which she is 
most at home, but even try to victimise the identical 
pair of birds which fostered her herself ? 
Conditions favourable to Regularity of 
Laying 
I may now conveniently discuss the reservations 
1 made above in stating the conditions under which, 
in my experience, a Cuckoo tends to break her 
rule of laying in nests of the same species of 
fosterer. 
I have been asked why I attached so much 
importance to trying to ascertain whether Cuckoo A 
actually did lay an egg on the days when she should 
