Aim: A reduction in dispersal ability has been thought to be a general trend for island land birds because of their physical isolation to a limited area, scarcity of predators, and ability to forage without flight in island habitats. However, recent studies have indicated that long-distance dispersal among island habitats may be maintained in island pigeon populations, which may be related to unstable food resource availability on a single island. Here, we investigated whether pigeons could maintain their long-distance dispersal ability to move among islands within the archipelago, even in highly insular conditions, by comparing two subspecies of Japanese wood pigeon (Columba janthina) distributed on islands with different levels of insularity.

Location: The Ogasawara, Izu and Nansei Islands in the north-western Pacific Ocean

Taxon: Japanese wood pigeon (Columba janthina)

Methods: We compared morphological traits related to dispersal potential between the two subspecies: C. j. janthina and C. j. nitens. We performed phylogenetic and population genetic analyses based on genomic markers to indirectly assess dispersal ability via the rate of gene flow and determine how long C. j.nitens in a more insular habitat have been isolated for a long time.

Results: C. j. nitens had a wing shape that was better suited for long-distance flight than that of C. j. janthina. Both subspecies maintained sufficient gene flow among the islands within their habitat archipelagos, with no evidence of isolation by distance. The divergence time of the two subspecies was 0.67 million years ago, which was sufficient to reduce dispersiveness compared to other cases of island land birds. 

Main conclusion: We showed that long-distance flight can be maintained under highly insular conditions despite long-term isolation from the ancestral populations. This indicates that the strategy of maintaining dispersal ability may also be possible in island systems, where organisms have repeatedly and globally shown reduced dispersal ability. Although further studies are required to reveal its evolutionary processes, our study provides an exceptional case for island birds' biogeography.