176 THE ZOOLOGIST. 



WHAT IS THE REASON OF THE GREAT 

 VARIATION IN CUCKOOS' EGGS?* 



By Dr. E. Rey, Leipzig. 



Translated and communicated by W. Wells Bladen, Vice-President, North 

 Staffordshire Field Club. 



Among the many attempted explanations of the great variation to be 

 found in the colouring and markings of Cuckoos' eggs, the application of the 

 Darwinian theory of selection seems at first sight to be most feasible. But 

 on closer examination it does not appear to be in accordance with many 

 well-founded facts. This theory supposes that those female Cuckoos whose 

 eggs most resemble those of the nest chosen, have greater chance for the 

 preservation of their offspring than others, whose eggs would be more liable 

 to be destroyed by the foster-parents. As it may moreover be presumed 

 that their daughters would lay eggs similarly coloured, and would make the 

 same selection of nests for their offspring as their mothers, the consequence 

 would be a preponderance of females whose eggs are similar to the nest 

 eggs in colouring, whereas the other class would become more rare, and 

 finally disappear. 



I have already demonstrated in my work on the Cuckoo, and am now 

 able to confirm by material at my disposal, and by nearly 2000 examples, 

 that eggs matching those of the foster-parents are only to be found in a 

 small percentage of cases. Those who only take into consideration the few 

 examples iu which Cuckoos' eggs are coloured like the nest-eggs, would 

 alone venture to ask that the theory of selection should be accepted ; whereas 

 others, who consider the question in its entirety, will reject it as untenable, 

 as far as the Cuckoo is concerned. 



If the resemblance of its eggs to those of the foster-parents were such 

 an advantage to the Cuckoo it would not be found as an exception to the 

 rule, but would, on the contrary — at least here in the neighbourhood of 

 Leipzig — be very perceptible in connection with Lanius collurio, most 

 Cuckoos' eggs (84 per cent.) being found in the nests of these birds ; of 

 282 Cuckoos' eggs found in the nests of the Red-backed Shrike, only 

 sixteen, about 5 per cent, were of the type of Lanius eggs. 



* "Was ist der Grund fur die grosse Variabilitat der Kuckuckseier ? " 

 Ornith. Monatschrifte des deutschen Vereins z. Schutze der Vogelwelt. 

 Jahrgang 1895. Nr 1. 



