Death Cause of Eastern Red-Rumped Swallows (Cecropis daurica) in a Mass-Mortality Event in Autumn Migration from Southern China

Bird migration is an awe-inspiring phenomenon that causes massive global shifts in bird distributions twice a year. To understand the evolution of this phenomenon, it is crucial to know the mortality costs of these journeys. Extreme weather-related events can lead to abnormally high mortality rates among migratory birds, while high mercury concentration may reduce the survival of songbirds in the field, especially for the long-distance migrant insectivores. Yet the specific vulnerability factors remain largely unknown. We collected the opportunistic dead Eastern Red-rumped Swallows (Cecropis daurica) in a mass-mortality event caused by a cold wave in autumn migration in Southern China. Mercury concentration in their tail feathers is 0.57 ± 0.37 µg g−1, lower than the established toxicity threshold for birds. The claws’ hydrogen stable isotopic (δD) values ranged from −116 to −78 ‰, with a mean of (−88.00 ± 8.22) ‰. Stable hydrogen isotopes indicated broad origins for the Eastern Red-rumped Swallows, ranging from ~30° N to ~62° N and ~10° E to ~150° E. Considering subspecies ranges, most of the dead swallows likely came from their almost furthest breeding sites. Our results indicated the primary cause of the mass-mortality event was likely fatigue or starvation resulting from long-distance flight during an extreme cold wave. Mercury exposure may not be the main direct cause of death.