Habits of the Cuckoo, 293 



tion it is intended to practise. Though very similar in some 

 instances to those of the skylark, there is a character about it 

 peculiar to itself, by which it may be readily known : its shape 

 is more oval ; it is also, in most instances, marked with minute 

 black dots." 



In some of the instances in which man or the cuckoo has 

 added to the eggs of certain birds an egg or eggs of another 

 kind, these birds have been known to reduce the total num- 

 ber, and, in the reduction, to retain the alien eggs and dis- 

 card their own. In V. 728. an instance of the buzzard's 

 discarding its own egg from among the eggs of poultry, which 

 it was incubating, is stated. Another is described by Professor 

 Rennie in his Habits of Birds, p. 145. ; it is this : — A few days 

 ago we had brought to us three eggs of the wood wren (Sylvia 

 sibilatrix Bechstein) ; and, being anxious to have them hatched, 

 we introduced them, after warming them slightly, into the nest 

 of a canary, then sitting upon four eggs of her own. In the 

 course of the day, two of her own eggs had disappeared, hav- 

 ing, we inferred, been destroyed by her, because she could 

 not cover the seven so as to keep them at a uniform tempera- 

 ture, the three small eggs being nearly equal in size to the 

 two which were gone." Professor Rennie has added : — "It 

 is, no doubt, for the same reason, that the birds in whose nests 

 the cuckoo parasitically deposits her egg, often, if not always, 

 turn out or destroy their own to make room for hers." 



The fact and suggestion cited from Professor Rennie are 

 sufficient, probably, to account for birds reducing the number 

 of eggs in their nests, when this has been rendered super- 

 numerary ; but it does not account for their retaining the alien 

 eggs and discarding their own. The wood wren's eggs were 

 individually smaller than the canary's, and might be disposable 

 more compactly ; the cuckoo's egg is, it is shown above, 

 slightly larger than the egg of any species of bird into whose 

 nest it is known to deposit it. Upon what principle, then, 

 are the alien eggs retained, and the proper ones ejected ? 

 Probably upon that of accident only. Professor Rennie, in 

 his idea, that " the birds in whose nests the cuckoo parasiti- 

 cally deposits her eggs, often, if not always, turn out or de- 

 stroy their own to make room for hers," can only refer to 

 instances in which the egg or young of the cuckoo has been 

 found in nests. It may be that the instances, never known of 

 by man, of the cuckoo's depositing it eggs in nests from which 

 the owners of that nest discard it, may be more numerous 

 than those instances, known of by man, in which the owners 

 of the nest retain it. If we take the natural number of the 

 pied wagtail's eggs at five, an instance is noted in V. 675., 



