THE CUCKOO'S SECRET

[To the Editor of THE SPECTATOR.] SIR,—In your issue of February 9th, 1934, Sir W. Beach Thomas again raised the age-old controversy as to how cuckoos deposit their eggs, mentioning a wager offered by another correspondent. It is well known that the making of wagers or offering monetary awards is taboo among scientific Ornithologists. It is also well known that I have refused to be associated with offers of this nature, and that I do not accept the fantastic theory of normal oviposition—i.e., the laying of the egg by the cuckoo direct .into the nest of the fosterers and it should have been obvious to those who were responsible for supporting such a theory how tremendously important it is, in such cases, to test the strict admissibility of the evidence.

The results of my investigations, extending over thir- teen years, have convinced me that there is only one method of deposition—regurgitation, i.e., the eggs are first oviposited on the ground or on old nests. There they rest until the cuckoo has found a nest in the right condition for their reception. They are then taken up by the bill and swallowed into the gullet or oesophagus, where they again rest until regurgitated into the nest, among

the eggs of the fosterer. •

To those who have studied the whole question in its minutest details, and have personally witnessed from hides, depositions into the nests of pied wagtails and reed warblers, there is nothing extraordinary in the habit of regurgitation. Cuckoos are inveterate egg stealers, which eggs they in- variably swallow whole ; they also regurgitate undigested food in the form of pellets or castings. That the habit of regurgitation during the course of evolution has been evolved from these various habits surely Cannot be denied. Many birds feed their young by regurgitation. True, there is a

vociferous minority who have been slow to accept regurgita- tion, and- ever ready to dish up new and impossible theories, and even to suppress conclusive evidence that was against them.

In the following report of my remarkable experiences it will be found that I have finally proved, beyond the bounds of controversy, that the eggs are deposited by regurgitation, via the bill ; and I do modestly claim that the evidence is by far the most important and conclusive that has ever been,-or can be, brought forward.

On June 16thI visited the South Kent marshes for the purpose of two weeks' intensive cuckoo watching. By the evening of the

18th I had located no less than twenty-five pairs of reed warblers and their nests in all stages, i.e., some with fresh eggs, others with incubated eggs and young. The territory was ideal, though rather

extensive for one cuckoo parasitic on reed warblers. There were a number of trees, willow, thorn and elm, all suitable and well situated as ' look outs' for the cuckoo, one pollarded willow being the most favoured.

The cuckoo, judging from her erratic behaviour, was a juvenile, i.e., a last years bird. On June 17th I found one of her eggs in a

read warbler's nest with two eggs apparently quite fresh. On

June 18th I found three, all in reed warblers' nests, one of which was on the point of hatching. On June 24th we found her fifth egg,

which she apparently deposited on the previous evening after we had made a final inspection of the nest at 6 p.m., before our depar- ture from the territory. Just' previous to this she had been seen in the willow close to the nest. Being so irregular, it was practically impossible to forecast depositions with any degree of certainty and she was evidently not a prolific layer.

On June 26th, in pushing my way through a thick reed bed, I came across an old moorhen's nest on which, to my great surprise,

two cuckoo's eggs were resting—one in the centre, the other on the edge. I at once recognized them as belonging to the cuckoo in possession of the territory. From previous experience, I was con- vinced that this was only a temporary resting place, consequently I carefully .marked them, with ordinary lead pencil, 6 ' and 7 ' respectively, and replaced them in their original position.

On the morning of June 27th I made an immediate inspection of the moorhen's nest, and found the cuckoo's eggs resting where I

had left them and, as the cuckoo was about, prepared for a long

watch—with the reed bed in which the moorhen's nest was situated and the pollarded willow in full view. About 2 p.m. she came over and settled in the willow and almost immediately planed down into the reeds where the moorhen's nest was situated. In leas than a minute she returned to the willow and I had her in full view (with powerful 12 times magnification prismatic glasses) sitting on a dead branch, and I could plainly see that the bill was closed.

There she sat motionless for half an hour, facing down the reed bed where there was a reed warbler's nest (previously inspected) with three fresh eggs. At 2.45 p.m. she glided down to the nest

and in less than eight (counted) seconds she flew up from the reeds and away over the meadow, bubbling loudly four times. I then

made an immediate inspection of the nest in which there were now two reed warbler's eggs and the cuckoo's I had marked 7,' the number being easily legible though much rubbed after having

rested more than half an hour in the gullet. I then made an inspec-

tion of the moorhen's nest and found egg marked 6 ' undisturbed. On June 28th I inspected the moorhen's nest and found egg marked 6' resting where left. After inspecting all warblers'

nests again, I spent the remainder of a fruitless day's watching near the pollarded willow. On June 29th I went straight to the territory and inspected the moorhen's nest, and found that egg marked '6 ' had disappeared. I then inspected all known warblers' nests, but without result.

On June 30th I made a final effort to find egg marked 6 '. My first visit was made to that part of the territory situated at least four to five hundred yards from the pollarded willow where I found

two cuckoo's eggs by the same bird on the 18th. One rebuilt nest

I had so far failed to locate. After a careful search I found the nest

containing three warbler's eggs and a cuckoo's on which I could just see my figure 6.' I was not altogether surprised ; at the same time it was .a very happy ending to a remarkable and unique experience, in further proof of the habit of regurgitatiot —and definite proof that the eggs were not oviposited into the warblers' nests! "

—I am, Sir, &c.,

PERCY F. BETNYARD, F.Z.S., M.B.O.U.