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 changes in bird distributions twice yearly. Understanding 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 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 caused by a cold wave in autumn migration in Southern China. Mercury concentration in their tail feathers is 0.57 ± 0.37 ug g⁻¹, significantly lower (P<0.05) than 1.95 μg g⁻¹, found in the feathers of migrating sea birds, which was identified as the cause of mortality among those 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, 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 maybe not the main direct cause of death.