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Cold-Wave Swallow Deaths Linked to Possible Exhaustion After Long Migration

Eastern red-rumped swallow perched on a wire in Kairwaan, Dehradun, India
Research · Ecology

More than 5,000 eastern red-rumped swallows (Cecropis daurica) were estimated to have died during a cold wave in Fangchenggang, Guangxi, China, in November 2017. A study published in 2026 examined 83 intact carcasses collected from the event to investigate whether mercury exposure or the energetic demands of long-distance migration could help explain the mass mortality.

The researchers measured total mercury in tail feathers and stable hydrogen isotopes in claw tips. Feather mercury ranged from 0.13 to 1.58 µg/g dry weight, and 89% of samples were below 1.0 µg/g; the overall pattern was below levels previously associated with harmful effects in birds. Hydrogen-isotope patterns vary geographically in precipitation and can become incorporated into animal tissues, allowing them to provide broad clues to where a bird had previously grown tissue. The claw-isotope results indicated a wide range of possible breeding origins.

The birds had likely completed a long migration

The sampled birds belonged to the subspecies C. d. japonica. When the isotope results were considered together with this subspecies’ breeding range, the researchers concluded that many of the dead swallows were likely to have come from distant northern breeding areas and had already completed a long southward journey before encountering the cold wave. These are probabilistic estimates: the isotope model combined calibration information from European swallows with an East Asian reference dataset from a different bird species because no dedicated calibration existed for eastern red-rumped swallows across their Asian breeding range. Hydrogen isotopes alone also cannot pinpoint longitude.

Possible breeding origins inferred from claw isotopes. Colours show relative likelihood; the red triangle marks the collection site.
Possible breeding origins inferred from claw isotopes. Colours show relative likelihood; the red triangle marks the collection site. Zhou et al. (2026), Figure 4. CC BY 4.0.

Exhaustion and cold best explained the mortality

The authors therefore considered fatigue or starvation after long-distance flight, compounded by the extreme weather, the most likely main explanation for the mortality, while mercury was unlikely to have been the primary direct cause. They also note an important limitation: most carcasses had been found crushed on roads and no necropsies were carried out, so there were no direct measurements of fat reserves or muscle condition to confirm starvation or exhaustion.

Sources and image credit

  1. Zhou, X., Fang, F., Huang, J., Liu, R., & Jiang, A. (2026). Death cause of Eastern Red-Rumped Swallows (Cecropis daurica) in a mass-mortality event in autumn migration from Southern China. Birds, 7(2), 25. https://doi.org/10.3390/birds7020025

Image: Satdeep Gill / Wikimedia Commons — eastern red-rumped swallow (Cecropis daurica) at village Kairwaan in Dehradun district, India, 28 March 2022. Illustrative photograph, not one of the birds from the Guangxi mass-mortality event. · CC BY-SA 4.0 · Source

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