Seasonal distribution of the Striated Heron (Butorides striata) in southern South America: evidence for partial migration

Abstract

The nominate subspecies of the Striated Heron (Butorides striata striata) inhabits most of South America. Previous authors suggested that the southernmost populations are partially migratory based on anecdotal evidence. An analysis of 9352 eBird records from south of 20°S reveals that the proportion of Striated Heron records during the austral winter months (June–August) decreases south of 24°S. Breeding occurs as far south as 37°S, whereas individuals have wintered as far south as 39°S. The seasonal distribution data strongly suggest that populations south of 24°S are partially migratory, with most individuals departing during the winter period in southern Paraguay, southeastern Brazil, Uruguay, and most of Argentina. Given the absence of band recovery, geolocator recovery, or satellite tracking data, it is unknown how far north the southernmost populations migrate, but heat maps of 35304 eBird records in South America suggest most individuals winter in eastern Bolivia and central Brazil. Further studies are needed to elucidate the migratory pathways and destinations of migrant individuals of the Striated Heron.