554 Mr. C. F. M. Swvnnertoii on 



probable cases a study of resemblances in birds' eggs may 

 help us a little to understand their coloration." The two 

 examples actually suggested above are intended rather to 

 illustrate a possibly useful line of contributory explanation 

 than to be regarded as put forward with any real feeling of 

 confidence in their individual validity. 



6. Parasitic Cuckoos' Eggs. — Mr. Stuart Baker, whose 

 field experience in this matter is probably quite unrivalled, 

 considers that it amply supports the theory of Dr. Baldamus. 

 1 have myself, in the past, been somewhat sceptical of the 

 part therein implied to have been played by the foster-parent 

 in eliminating Cuckoos' eggs unlike her own; this is in view 

 of the fact that as a schoolboy I sometimes replaced an egg 

 I required with another of quite different appearance, or 

 even with a rounded stone, and the old bird would go on 

 sitting. Mathews (' Birds of Australia ') quotes an instance 

 in which Petrels were found still sitting on their empty nests 

 a week after the removal of the eggs. But, if we regard, the 

 tendency to reject suspicious-looking eggs as selection's 

 natural reply to special victimization by Cuckoos, this 

 " readiness to sit on anything " may have meant no more 

 than that the species tested had not been greatly victimized. 

 Under these circumstances, mimicry in Cuckoos' eggs might 

 well, after all, often have been brought about by the action 

 of the host alone. The finding of deserted nests containing 

 a Cuckoo's egg certainly seems to support this view, but 

 the question must still, I think, be asked : " Was it detection 

 of the Cuckoo's egg that caused the desertion?-" At any 

 rate, the part played by the foster-parent in the elimination 

 of unconvincing substitutes can be very easily tested by 

 experimentation on wild birds. 



In any case, whenever the foster-parents' eggs were more 

 or less low-grade, the discriminative action of enemies might 

 well also have come into play, and have contributed to the 

 selection of the resemblance. An egg, little known (or 

 known to be pleasanter), and unlike the others in the nest, 

 would be quite liable to special testing even where these 



