Breeding population size and trends of American Oystercatchers on small islands of Bahía Santa María-La, Reforma, Sinaloa, Mexico

The islands of Bahía Santa María-La Reforma, Sinaloa, Mexico, host the largest breeding population of the western race of American Oystercatchers (Haematopus palliatus firazari). This resident shorebird has a small breeding range restricted to northwestern Mexico and, although listed as endangered, little is known about their breeding biology. We estimated the number of pairs on the El Rancho and Melendres islands, the detection rate of pairs in different habitats, and the relationship between habitat length and number of pairs during three breeding seasons (2016-2018). In 2016 and 2017, the number of pairs peaked in mid-April (97 and 95 pairs, respectively) at El Rancho, and in early May and early April (50 and 42 pairs, respectively) at Melendres. In 2018, both islands had the maximum number of pairs during mid-May, with 109 pairs at El Rancho and 42 at Melendres. Detectability of breeding pairs in plots with sandy beach and dune habitats was higher (86-100%) than in those with mangrove habitat (22%). Detectability was associated with habitat type and should be considered in estimating the population size of this subspecies. We estimated that ~ 11% of the total firazarì American Oystercatcher population breeds on these two small islands. The density of breeding pairs varied between habitats (range = 0-13 pairs/km). We also found a positive relationship between the number of pairs and plot length, suggesting that habitat availability might be a limiting factor for populations of American Oystercatchers in Mexico. Las islas de la Bahía Santa María-La Reforma, Sinaloa, México, albergan la población más grande de la raza oeste del Ostrero americano (Haematopus palliatus firazari). Esta ave costera residente tiene un pequeño intervalo reproductivo restringido al noroeste de México y, aún cuando está registrada como en peligro, poco se sabe de su biología reproductiva. Estimamos el número de parejas en las islas El Rancho y Melendres, la tasa de detección de parejas en diferentes hábitats, y la relación entre el tamaño del hábitat y el número de parejas durante tres temporadas reproductivas (2016-2018). En 2016 y 2017, el número de parejas alcanzó un máximo a mediados de Abril (97 y 95 parejas, respectivamente) en El Rancho, y en la primera semana de Mayo y de Abril (50 y 42 parejas, respectivamente) en Melendres. En 2018, ambas islas tuvieron su máximo número de parejas a mediados de mayo, con 109 parejas en El Rancho y 42 en Melendres. La detectabilidad de p