Summary

Humans have drastically reduced avian diversity, with the majority of extinctionsoccurring on islands. Previous studies have quantified various aspects of this decline,including both taxonomic and phylogenetic diversity loss due to recorded extinctions.Other studies have estimated that unrecorded island bird extinctions – those that leftno known fossil evidence – may represent hundreds of additional losses. However,these analyses have only focused on species diversity. In this paper, we bridge these tworesearch efforts by estimating the phylogenetic diversity lost due to unrecorded islandbird extinctions. Our findings suggest that the loss of phylogenetic diversity may besubstantially smaller than expected, given the number of extinctions. Our results sug-gest that while unrecorded extinctions probably represented around 60% of all speciesextinctions, the majority of the phylogenetic diversity loss was likely caused by therecorded extinctions. The reason for this is that while extant island endemics are onaverage slightly more phylogenetically distinct than expected by chance, a dispropor-tionate number of unrecorded extinctions are predicted to have been from islands inthe eastern Pacific. Extant birds from this region generally have lower phylogeneticdistinctiveness than other birds and the extinct species therefore likely did as well.