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