There seems to be an error in the math. EOO = 87,600 km2. Four birds per km2 = 350,400 birds, times 0.1 (10% of mapped range occupied) = 35,040 birds – the minimum pop size estimate. Sounds like the EOO estimate that went into the pop size calculations above is smaller than 87,600 km2.
Be that as it may, I do not believe that a population density estimate from O. leucopleurus in Argentina is representative for O. adela just because they are congeners. For starters, they differ in natural history in that O. leucopleurus is a partial austral migrant (see Herzog et al. 2019, Birds of Bolivia Field Guide) and O. adela a year-round resident. In Bolivia, O adela is not only patchily distributed, but also quite rare in general. Perhaps in the southernmost of the 3 range polygons it is more common, but anywhere else I would think that a density of 4 birds per km2 is pretty high, let alone 12 per km2.
In conclusion, using a density estimate of a congener as a yardstick for this Bolivian near-endemic seems ill-founded to me and likely overestimates the population size of O. adela.
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