Is the resident subspecies of the Eurasian spoonbill in Mauritania, <i>Platalea leucorodia balsaci</i> , threatened by introgression of genes from the migratory nominate subspecies <i>leucorodia</i> ?

Abstract Throughout its range, the Eurasian spoonbill Platalea leucorodia is migratory, but there is a well-documented exception in a population living in the Parc National du Banc d’Arguin in Mauritania. Based on their smaller body size, absence of a yellowish breast band during breeding, and fully black bills, they were assigned subspecies status ( Platalea leucorodia balsaci ) in 1974. Despite obvious threats (small numbers, and their low-lying breeding islets being under pressure from sea level rise), the Mauritanian spoonbill has not been assessed for inclusion on the IUCN Red List. The nominate subspecies P. leucorodia leucorodia , which joins the Mauritanian subspecies at Banc d’Arguin during the non-breeding season, is categorized as Least Concern. There is genetic and behavioural evidence of gene flow between balsaci and leucorodia , and in December 2023 and 2024, we observed that over half of the spoonbills born at Banc d’Arguin (identifiable by their colour-rings) had the yellow-tipped bill characteristic of leucorodia . As the increase in the marked balsaci population has not resulted in resightings away from Banc d’Arguin in the well-monitored flyway of leucorodia from West Africa to Europe, the introgression of leucorodia genes into balsaci seems to have left the isolating character of residency of the Banc d’Arguin-breeding spoonbills intact. We propose that the subspecies balsaci should be categorized as Vulnerable on the IUCN Red List. We recommend continued monitoring of the breeding population of balsaci , with the inclusion of new studies of morphology and genetics.