# The Zoologist/4th series, vol 3 (1899)/Issue 694/What is the Reason of the Great Variation in Cuckoos' Eggs, Rey

Published in The Zoologist, 4th series, vol 3, issues 694 (April, 1899), p. 176–178
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 in 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.
​Quite irrespectively of this, how would it be possible to explain, by means of the theory of selection, the fact that there are a great number of Cuckoos' eggs which have a particular type of colouring not to be found in any eggs known to us, and others marked like eggs with which eggs of the Cuckoo are seldom placed. We must therefore cast about for another explanation. In a number of species of birds we see that the eggs differ considerably in colour and marks when they come from places far apart. To quote a few examples: eggs of Phylloscopus trochilus from Lapland are, contrary to those found in our parts, marked with dark spots, so dark as almost to be mistaken for eggs of Phylloscopus rufus. Again, whilst spotted eggs of the Redstart are rare here, examples are frequent in high northern latitudes; and whereas Caccabis saxatilis lays distinctly spotted eggs in the alpine regions, its eggs from Greece are monochromous, or but very slightly marked.
Now, as Wickmann has demonstrated that eggs take their colour from the transposing products of the blood, so must we lead back the varieties of colouring to the variety of these transposing products, and the latter again to the chemical or physical properties of the blood. We must look upon food as the chief cause of the difference in the formation of the blood, for according to its different chemical properties it will produce lesser or greater variety in the composition of the blood. We must therefore take, as the cause of the variation in the colouring of the eggs of the same bird from different places, the difference of food according to the place of their residence. Not that different nourishment would produce an immediate change in the colour of the eggs—for we know that every female bird will, during its whole life, unless pathological changes should occur, lay the same, or at least very similarly, coloured eggs—but the difference in food will, in the young female bird, whilst the body is developing, have an abiding influence upon its blood-forming organs, and determine the colour of her future eggs. It is clear that apparently similar food can produce different results, for we often see that insects and larvæ, externally alike, have, chemically, quite different bodies; and, again, quite distinct insects are chemically alike.
If, on the one hand, the variation in the eggs of different female birds of the same species is occasioned in this way, the law of heritage confines it on the other. We see that Shrikes and Pipits lay very different eggs, but notwithstanding the number of varieties there is a decided type running through them all. Here we see a certain inherited resemblance, whereas in other cases the eggs are so completely distinctive as to be unrecognizable. If we apply this to the Cuckoo, we are not astonished if almost every bird lays differently coloured eggs, because the difference of food arising from the various foster-parents, according to their kind and individuality, pro​duces a much larger variety than in other birds. And if we further apply to the Cuckoo the law of heritage, over and above the difference in food, the variation in the eggs would be enormously increased. Considering the manifold variety thus produced, it is quite possible that the eggs of the Cuckoo should assume a likeness to the eggs of other birds, even of such as it does not choose to lay with. We must also admit that the principle that the food of many birds, though it may not affect their own eggs, has its influence on the colouring of the eggs of their offspring, can also be applied to the Cuckoo, in the case also when it is nurtured for generations in the nests of the same species of birds whose eggs do not vary much.
We can, with some amount of certainty, assume that our Cuckoo, before he became a nesting parasite, laid monochrome blue eggs, as we see now in its near relatives the North American Coccyzus americanus and C. erythrophthalmus, which have already occasionally begun to give up rearing their own young. The blue eggs of the Cuckoo, exclusively found in the nests of the Redstart, which also lays blue eggs, may be traced to similarity of food and inheritance.