Evidence for longitudinal migration by a “sedentary” Brazilian flycatcher, the Ash-throated Casiornis

The digitalization of museum collections and concurrent increase in citizen-scienceinitiatives is ushering in an era of unprecedented availability of primary biodiversity data. These changespermit a reappraisal of phenological patterns of tropical species. I examined spatio-temporal variation in thedistribution patterns of an ostensibly sedentary endemic Brazilian flycatcher, the Ash-throated Casiornis(Casiornis fuscus), using both specimen data from museums and sighting records and rich media data fromcitizen-science initiatives. I found compelling evidence for partial intratropical longitudinal migration toAmazonia and the Cerrado biomes from the species’ core range in the semi-desert Caatinga biome andadjacent ecotones. These records from outside of the Caatinga were distributed during the height of the dryseason from April to October, although the Caatinga is not entirely vacated at this time. This pattern ofpartial migration leads to a doubling of the distributional range of Ash-throated Casiornises and stronglysuggests that the species is a breeding near-endemic of the Caatinga biome. This pattern was potentiallypreviously not apparent because of significant biases in specimen collection between biomes, giving a falsesense of abundance in the Brazilian Amazon.