INTRODUCTORY 
9 
eggs would merely expose a lack of personal 
experience in this direction. 
Of course, in the case of our own study in 1920, 
the fact that after the laying of the tenth egg we had 
reached the stage when we were able to take friends 
out to see the Cuckoo lay her eggs at approximately 
appointed times on a given territory, conclusively 
proved that we were in fact correct in our assump- 
tion that the Cuckoo we were studying was one and 
the same bird. And this conclusion is most strik- 
ingly borne out by the wholly successful manner 
in which, in 1921, we were able, when so desired, 
to place the cinematographer just where he could 
film the Cuckoo laying. 
An earlier acquaintance with Rey’s monograph 
might have sooner turned my attention to the 
Cuckoo problem. For many a season, like most 
field-naturalists, I duly made an entry in my note- 
book of the day when the Cuckoo returned. Later 
in the year, the finding of an occasional egg in one 
or other of the nests of the small birds usually 
selected by the Cuckoo for fosterers was an event, 
interesting but virtually meaningless, for I had not 
grasped the truth that the way to an intimate know- 
ledge of the Cuckoo’s habits lies through an in- 
tensive study of its eggs in situ. 
