Distinctive and highly variable bird migration system revealed in Eastern Australia.

Our understanding of bird migration is heavily biased toward long-distance movements in the Northern Hemisphere,^1,2,3 with only fragmented knowledge from the Southern Hemisphere.^4,5 In Australia, while some species migrate,^4,6,7,8 the timing and direction of large-scale, multi-species seasonal movements remain critically understudied due to the complexity of movement in this region and a lack of research personnel and infrastructure.^7,9 It is still unclear whether there are pronounced and structured mass movements resembling those in the Northern Hemisphere.^10,11,12 Here, we analyze data from a latitudinal transect of weather radars spanning the entire coastline of Eastern Australia to determine the magnitude, directions, timing, and variability of bird migration compared to that of Northern Hemisphere migration systems. Bird movements exhibited sequential seasonal peaks along a latitudinal gradient with seasonally contrasting flight directions, confirming that a structured bird migration system exists. Three features were distinct from Northern Hemisphere migrations. First, distinct movements occurred around sunrise with comparable magnitudes to nocturnal migration, likely representing a strong diurnal component to the bird movements. Second, migration intensity averaged 0.06 million birds km^-1 in autumn, much lower than Northern Hemisphere migrations.^11,12,13 Finally, flight directions were more dispersed, and the timing and amount of migration were highly variable between years compared to Northern Hemisphere migration systems, perhaps in response to variable climate.⁷ This first quantification of continental-scale movements in Australia revealed a distinctive migration system, and it suggests that much remains to be discovered about the ecological and evolutionary factors shaping animal migrations in the Southern Hemisphere.