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Christie’s Images, Ltd.
Can you see the tiny bird at the end of this 18-karat gold pistol? It moves, rotates, shakes its beak — and sings.

Imagine the world’s most complicated mechanical cuckoo bird — hidden not in a clock but in what appears to be a double-barreled pistol.

Wind it like a watch with a key that’s stored underneath the barrel, and when it’s “fully loaded” use your thumb to pull back the safety lever. Then squeeze the trigger. What emerges is not a bullet but the tiny bird, no larger than a cherry pit. It rotates around its own axis, flaps its wings, shakes its tail and moves its two-millimeter beak. It even sings. (Unlike the cuckoo in a clock, it doesn’t tell time.)

On May 30, not one but a pair of identical singing-bird pistols, estimated to sell for 20 million Hong Kong dollars to 40 million Hong Kong dollars for the pair (US$2.5 million to US$5.1 million), will go on sale at Christie’s auction of watches.

“To be clear, this is not a watch or clock,” says Aurel Bacs, Christie’s international head of watches. “These are automatons.” A fascination with the mechanics of human and animal movement, which began in the Renaissance, developed in the 19th century into a movement “to recreate those mechanics ourselves,” he says. These pistols, which date back to roughly 1820, are attributed to the Rochat brothers, Swiss artisans who pioneered the art of mechanical singing birds.

Here’s why these strange items carry such a high price tag:

Complications: “We’ve played with Legos and understand wheels and levers, but it’ll take us a lifetime to understand a complication like this,” says Mr. Bacs. Every movement is mechanically engineered — there are no batteries.

In a typical cuckoo clock, a bird is pushed out the door by a folding ladder and does not move its body. But inside these singing birds, covered with real feathers that are intricately dyed and decorated, are skeletons that allow them to move their body parts.

Size: It matters. Watchmakers show their prowess not in large clocks, but in the intricate work of a tiny timepiece. A cuckoo clock is 20 to 50 times the size of than the 15-centimeter-long pistol, and the small scale of the singing birds is a testament to delicate handiwork.

Shape: This matters, too. “Most of the times, you’ll find [a singing bird] in a snuff box or bulky clock, where the blocky form is dictated by the function,” says Mr. Bacs. “But the pistol is a particularly awkward shape. That makes it far more challenging.”

The playlist: “A cuckoo bird does a repetitive ‘cuckoo cuckoo,’” says Mr. Bacs. “But the melody this bird sings is completely organic and spontaneous. It’ll never repeat the same tweets.” On top of that, the song lasts for 20 seconds.

Rarity: This is the only pair of identical singing-bird pistols known to exist in the world. “We believe this pair of pistols was made for the royal courtesans in China, where a matching pair was extremely popular,” says Mr. Bacs. Indeed, there are only four other individual pistols known of: one in the Museum for Islamic Art in Jerusalem, another in the Maurice Sandoz Collection in Switzerland, and two in the Patek Philippe Museum in Switzerland.

“In the world today we have more and more billionaires who realize that they can buy a Mercedes, a tailored suit or even a house anytime they want,” says Mr. Bacs. “But items like this don’t come on the market often.” Christie’s hasn’t sold a bird pistol since 1989, and that one went for just over US$1 million. (It’s one of the two now in the Patek Philippe Museum.)

Materials. The pistol itself is made from 22-karat gold and adorned with diamonds, pearls and enamel—though, as Mr. Bacs puts it, “You almost forget about the expensive materials that made up the pistol. It’s the ensemble that matters.”

To see a video of the singing bird pistols at work, click here.

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