436 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



vours; but after a little respite, the work is resumed, and goes on almost inces- 

 santly till it is effected. It is wonderful to see the extraordinary exertions of the 

 young cuckoo, when it is 2 or 3 days old, if a bird be put into the nest with it 

 that is too weighty for it to lift out. In this state it seems ever restless and un- 

 easy. But this disposition for turning out its companions begins to decline from 

 the time it is 2 or 3 till it is about 12 days old, when, as far as I have hitherto 

 seen, it ceases. Indeed, the disposition for throwing out the egg appears to 

 cease a few days sooner; for I have frequently seen the young cuckoo, after it 

 had been hatched Q or 10 days, remove a nestling that had been placed in the 

 nest with it, when it suffered an egg, put there at the same time, to remain 

 unmolested. The singularity of its shape is well adapted to these purposes; for, 

 different from other newly-hatched birds, its back from the scapulae downwards 

 is very broad, with a considerable depression in the middle. This depression 

 seems formed by nature for the design of giving a more secure lodgement to 

 the egg of the hedge-sparrow, or its young one, when the young cuckoo is 

 employed in removing either of them from the nest. When it is about 12 

 days old, this cavity is quite filled up, and then the back assumes the shape of 

 nestling birds in general. 



Having found 'that the old hedge-sparrow commonly throws out some of her 

 own eggs after her nest has received the cuckoo's, and not knowing how she 

 might treat her young ones, if the young cuckoo was deprived of the power of 

 dispossessing them of the nest, I made the following experiment. July Q. A 

 young cuckoo, that had been hatched by a hedge-sparrow about 4 hours, was 

 confined in the nest in such a manner that it could not possibly turn out the 

 young hedge-sparrows which were hatched at the same time, though it was 

 almost incessantly making attempts to effect it. The consequence was, the old 

 birds fed the whole alike, and appeared in every respect to pay the same attention 

 to their own young as to the young cuckoo, till the 13tbj when the nest was 

 unfortunately plundered. 



The smallness of the cuckoo's egg, in proportion to the size of the bird, is a 

 circumstance that hitherto I believe has escaped the notice of the ornithologist. 

 So great is the disproportion, that it is in general smaller than that of the house- 

 sparrow; whereas the difference in the size of the birds is nearly as 5 to 1. I 

 have used the term in general, because eggs produced at different times by 

 the same bird vary very much in size. I found a cuckoo's egg so light that it 

 weighed only 43 grs., and one so heavy that it weighed 55 grs. The colour of 

 the cuckoo's eggs is extremely variable. Some, both in ground and penciling, 

 very much resemble the house-sparrow's; some are indistinctly covered with 

 bran-coloured spots ; and others are marked with lines of black, resembling in 

 some measure the eggs of the yellow-hammer. 



