The Common Cuckoo's habit of laying
its eggs in the nests of other birds is well known, together with the curious
fact, that although it is a reasonably large bird, measuring more than
a foot in length, its egg is not more sizeable than that of the little
birds, such as the hedge-sparrow, robin, or redstart.
The hen cuckoo seems
to take much care in choosing a nest where she will leave her egg.
Sometimes the chosen nest may be so small in comparison to the hen's size
that it would be extremely difficult for her to actually lay the egg directly
in the nest. To overcome this problem the female cuckoo has been
seen to lay the egg on the ground, then carry it up to the chosen nest
in her bill. One of the earliest records of this being sighted was by two
sons of a Scottish farmer, a Mr Tripeny of Coxmuir. They reported
that they viewed on June 24 1838, the occurrence of a parent cuckoo transporting
her egg in this manner to another bird's nest. Later in that century,
a German forrester by the name of Adolf Muller, who worked at Gladenbach
in Darmstadt, reported viewing through his telescope a parent cuckoo laying
her egg on a bank, and carrying to the nest of a wagtail in her bill.
In the past, a few
cuckoos have been shot whilst actually carrying an egg of their own kind
in their bill, the egg itself probably being the bird's own. (The
earliest instance of this in the UK seems to be that reported by Thompson
(B. Irel. iii. p. 472); another was recorded in 1851 (Zool. p. 3145); but
Le Vaillant seems to have been the first to discover the same type of action
occurring in a South African species (Ois. d'Afr. v. pp. 47, 48); and although
he was often an untrustworthy witness, in this case he seems to have spoken
truly). This way that the mother cuckoo uses to carry its own eggs
is probably the source of the theory thay cuckoos suck eggs of other birds.
Many ornitholigists therefore believe the egg-sucking theory to be untrue.
In general however,
the act of the hen bird depositing her egg in a nest is very infrequently
winessed. She appears to use a lot of care in deciding where to place
the egg. Aelian, who flourished in the second century, declared (De
Nat. Anim. III. xxx.) that the Cuckoo laid eggs in the nests of those birds
only that produced eggs like her own - a statement which is of course far
too general; but in 1767 popularilty was given to this idea by Salerne.
Although Saleme did not believe in the hypothesis himself, a person living
in Sologne mentioned to him that a Cuckoo's egg has a similar colour to
the eggs which are usually found laid by the bird in whose nest it is deposited.
Doctor Baldamus inedependantly
idea, however it cannot be a universal truth, as there is no alikeness
of color between cuckoo's eggs and those of the Hedge-sparrow for example,
a bird in whose nest the cuckoo's eggs can frequently be found and whose
eggs of which are a bluish grey.
In the mid 1800s,
another German forrester, Herr Braune at Greize in Reuss (now in present-day
Thuringi) discovered an egg in the oviduct of a cuckoo, which he had shot
as the bird had had just exited from an Icterine Warbler's (Hypolais icterina)
nest. The egg itself was of a very similar colour to that of the
warbler's. Upon inspecting the Warbler's nest he discovered a simlar
egg which it is assumed that the cuckoo had just deposited.
In that same century,
a Herr Grunack discovered a particularly unusually colored egg, very dissimilar
in appearance to how a cuckoo's egg would normally look. On inspecting
the embryo inside, the distinctive zygodactyl feet were observed, indicating
that it was indeed a cuckoo's egg.
It has been suggested
that there may be a hereditary aspect in the choice of nest where the hen
cuckoo deposits her egg. It is quite possible that a cuckoo who has
succeeded in placing her egg in, for example a Reed-Wren or Titlark's nest,
may continue to use this type of bird's nest in the future. In 1873
it was reported that for an 8 or 9 year period a pair of wagtails who had
built their nest in virtually exactly the same location, had each year
fostered a cuckoo chick, and numerous parallel circumstances are thought
to have occurred. This habitual choice of the type of nest may pass
down in a hereditary fashion from mother to chick, resulting in the hen
daughter of a Cuckoo who had constantly chosen to deposit her egg
in to a Titlark's, Reed-Wren's or Wagtail's nest, choosing the same
type of nest as her mother. This habit, if continually successful
in resulting in the birth of live young, may then pass down through generations.
Although there may
be some difference between eggs which are laid by individual cuckoos, those
laid by the same cuckoo are quite similar even over many years, and it
is conceivable that a daughter cuckoo's eggs may have a similar appearance
to those of her mother's. Taking into consideration the idea that
a cuckoo's habit of selecting the nest of one particular species of bird
could (if resulting in a successfully brooded chick) become hereditary,
it has been suggested that the situation whereby a cuckoo's eggs
resemble those of the bird in which it chooses to place them could be explained
via a proces of natural selection....the cuckoos who inherit and transmit
the habit of placing their eggs in the nest of a bird whose own eggs are
similar being likely to thrive and survive, whereas those who did not inerit
this habit being likely to perish and thus stop the genetic line.