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Trollopes Bird Notes

June 2019

Click for Trollope's March 2019 notes

How the mystery of the Cuckoo’s eggs was solved by Chance

Nearly 100 years ago, in 1921, a 12-minute film stunned the world and filled cinemas on both sides of the Atlantic. A Cuckoo had been filmed laying an egg in a host bird’s nest, putting to bed a long debate in the ornithological world. There had been plenty of sightings of a Cuckoo with an egg in its mouth and one theory was that it laid its egg elsewhere and deposited the egg in the nest. The opposing theory was that the Cuckoo laid its egg in the nest and then took a host egg away so that the host bird wouldn’t notice. The former theory was backed up by the fact that a Wren, whose domed nest would make it very difficult for a Cuckoo to lay an egg, was sometimes the host. How this film was made, and the hundreds of hours of dedicated fieldcraft involved, makes a fascinating story.

It all started at Pound Green Common in the Wyre Forest in the West Midlands, where Edgar Chance ran a family chemicals business and was an obsessive egg collector who became fascinated with Cuckoos and their host species’ nests. In the summer of 1918, he and several colleagues found 14 parasitised nests and at the same time established that there were two female Cuckoos holding territories.

Because there were slight differences in background shade and markings between the two females, he was able to determine that ten eggs were laid by Cuckoo A and four by Cuckoo B. The host species was Meadow Pipit in all 14 nests, despite Skylark, Tree Pipit, Linnet and Yellowhammer also breeding on the common. Finding ground-nesting birds is extremely difficult as the nests are usually well hidden under tufts of vegetation. I have frequently flushed pipits and larks from their nests sites when walking through suitable habitat in the breeding season, without being able to find the nest, despite some determined searching; so I admire the fieldwork skill of Chance and his colleagues.

Chance collected all these nests and, by examining the embryos, he was able to determine that Cuckoo egg and host eggs were at the same stage of development and therefore the Cuckoo must be watching the nest to assess hatching times. With a Meadow Pipit laying on average four eggs (one a day) the Cuckoo has only a short window of opportunity to carry out its dastardly deed.

By checking nests every day, Chance found out a Cuckoo egg takes 12 days of incubation to hatch compared to a Meadow Pipit’s 13.

The following season Cuckoo A returned, much to Chance’s delight, and he found 18 parasitised nests that summer, again all Meadow Pipits. By monitoring these nests he was able to establish that Cuckoo A laid on every other day. Despite all this time in the field he didn’t witness an egg being laid. He assumed it must be very early in the morning, similar to when the pipits lay.

On checking as early as 3.45am, he found the Cuckoo egg already in the nest. Therefore, the Cuckoo must have been laying in the afternoon/evening and he and his colleagues concentrated on watching the female Cuckoo, who would be hidden in a tree, sometimes up to three hours at a time, maybe 100 yards away from the target nest. Most of her laying took place between 3pm and 6pm. She would glide down to the nest, remove an egg with her beak and lay her egg in the nest. The whole exercise took about ten seconds, after which she would fly back to her perch and consume the stolen egg.

Chance now had everything in place for possible filming. The next season, and to his great relief, Cuckoo A returned once again, and by using two hides and two cameramen he was able to film seven layings during the season. These were the main focus of the film which gripped the world in 1921. Cuckoo A laid her eggs on the following dates, aided by Chance ‘restarting’ some of the breeding attempts of the poor Meadow Pipits. (May 11, 13, 16, 18, 20, 22, 24, 26, 28, 30, June 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27 and 29). How the Cuckoo could keep track of so many nests to lay at the right time and how Chance managed to keep up with her deserves admiration on both counts.

All these nests and their contents were kept by Chance, with all the dates accurately recorded. Modern research has shown that Cuckoos will do their own restarting by consuming a clutch of eggs and then watch the host pair rebuild nearby. The record number of eggs laid by a Cuckoo without man’s help stands at 25.

It should be noted that Chance‘s fieldwork methods would be breaking present-day laws. In fact his obsession for egg collecting never left him and he collected clutches of some very rare birds and was eventually thrown out of the Ornithological Union.

Charles Trollope


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