Colonial and Non-Colonial Birds Breeding on Dredge-Spoil Islands in a Tropical Wetland in Mexico

Abstract. In 2012, 11 small islands were created by the deposition of sediment from dredging operations in connection with the creation of navigation channels, along the main canal of Bahía Tóbari, Sonora, Mexico. During the breeding seasons of 2017–2019 and 2021–2022, we conducted repeated boat surveys each season, and documented 12 species of birds breeding on these islands, including eight species of colonial waterbirds, three shorebirds, and one owl. The number of breeding species varied between 7 (2017) and 11 (2019), and the number of breeding pairs varied between 1,686 (2017) and 17,970 (2018). From 2017 to 2018, there was a ten-fold increase in the total number of pairs (from 1,686 to 17,970), then it decreased to 8,482 pairs by 2022. Royal Tern (94 fold) and Elegant Tern (31 fold) were the species whose numbers increased the most from one year to the next. Six of these species are federally listed in Mexico, and five of them were not previously recorded as breeders in the bay. The number of pairs of Elegant Tern, Black Skimmer, and American Oystercatcher are 10%, 15%, and 7% of their biogeographic population, respectively, and therefore this site qualifies as an Important Bird and Biodiversity Area.