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The are some fir tree forests in Mexico that are reputed to have butterfly populations so dense that their collective weight is sufficient to occasionally break off tree branches.

An example of this claim:

Monarchs living east of the Rocky Mountains in North America fly south each fall, gathering in central Mexico's Oyamel fir forest for the winter. Millions of Monarchs gather in the this forest area, covering the trees so densely that branches break from their weight. Scientists aren’t sure how the butterflies navigate to a place they have never been. No other population of Monarchs migrates this far.

Has this ever actually happened? If so, has it ever been photographed or captured on video?

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There is a first-person account from 1976, from Dr. Fred A. Urquhart, part of a small team that found the wintering site of the Monarch butterflies after many years searching.

While we stared in wonder, a pine branch three inches thick broke under its burden of languid butterflies and crashed to earth, spilling its living cargo.

Source: National Geographic

In a corresponding article in the Journal of the Ledpidopterists's Society, the authors describe a primitive means used to estimate the weight of the butterflies on a branch - by noting how much a branch bowed by the weight of the butterflies, removing the butterflies and trying to weigh it down equivalently with stones. Their system sounds to me rather inaccurate as it depends on matching the the centre of gravity of the butterflies, but they came up with an estimate of 3.8kg on a 1.5m branch.

That makes the idea that a small and weak branch could break under the additional weight of the butterflies seem plausible.

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