Learning About Western Osprey Nests in a Fishing Community of the Sonoran Coast
Learning About Western Osprey (Pandion haliaetus) Nests in a Fishing Community of the Sonoran Coast
By: Lucila Armenta Méndez and Manuel Muñoz Espinoza
The western osprey (Pandion haliaetus) is a permanent resident bird of prey along the coast of Sonora. Measuring close to 23 in size, with a 5.5 ft wingspan, it chooses to nest in very interesting places with close proximity to humans.
The western osprey is an apex predator, specially adapted to feed on fish caught in bodies of water near their nests. It’s distributed on all continents, with the exception of Antarctica (1).
The western osprey is recognized as an indicator species of the state of conservation of local aquatic ecosystems and proposed as a sentinel species to monitor the global effects of contaminants on wetlands, as their population decreased dramatically throughout North America during the 1960s and 1970s due to the adverse effects of organochlorine pesticides and other environmental pollutants, especially DDT (2) in their reproduction.
Thanks to conservation efforts, the species has recovered and since the 1970s, populations of western ospreys have increased in North America and the western Palearctic. For populations that nest in northwestern Mexico (Baja California coast, Gulf of California Islands, coasts of Sonora and Sinaloa) findings show stable growth in most of the area of study. However, for Sonora they decreased by 26%, from 214 couples in 1993 to 158 couples in 2006 (3). Although western ospreys have been well studied throughout their range, there is a need to advance our understanding of their ecology at the regional level and assess the availability of nesting sites and the consequences for their reproduction in the face of continued exposure to pollutants due to the development of agriculture and mining, which have reduced the quantity and quality of their habitat along the northwest coast of Sonora.
In addition, because western ospreys are a conspicuous and majestic species that live close to human communities, they are well-suited to study their reproduction patterns and implement outreach efforts, programs, and environmental education that produce strategies to manage and conserve the species.
This is why in December 2020, with funds provided by the UA-CONACYT Binational Consortium, also known as the Arizona-Mexico Consortium for Arid Environments (CAZMEX) and the support of community monitors, we began to study the nesting habits of western ospreys on the Northwest coast of Sonora, in the Ejido Rodolfo Campodónico. The goal was to involve the local community in the research project “Effects of pollutants and human activities on the reproduction of the western osprey (Pandion haliaetus) on the coast of Sonora” and enrich the knowledge of the “Grupo Lobos” community monitors with one more tool for their cultural, economic and inclusive benefit to protect and conserve the natural resources of the P. haliaetus nesting area.
This study site is located within the San Jorge Bay Wetlands Ramsar site, which extends along 23.6 linear miles of coastline in Northwest Sonora and is located south of the Alto Golfo de California and Colorado River Delta Biosphere Reserve and to the west of the Pinacate and Gran Desierto de Altar Biosphere Reserve. The area encompasses the wetland-terrestrial ecotone, which includes coastal dunes and some salt flats. Inland, the Sonoran Desert has experienced a substantial development of agricultural, mining, and tourist activities, including areas east and southeast of San Jorge Bay, predominantly ejido lands, part of the agricultural area of the Caborca valley, and a large mining operation that have modified the habitat and represent an imminent risk of exposure to pollutants for wildlife.
Western osprey nests
In the wetland-terrestrial ecotone of the coast of Sonora, the presence of columnar cacti, particularly cardon (Pachycereus pringelei) and saguaro (Carnegiea gigantea), provide safe nesting sites for P. haliaetus and are the preferred cactus to build their nests in.
The dominant cactus in the study area is the saguaro, associated with thickets of burro-weed (Ambrosia dumosa), creosote bush (Larrea tridentata), and mesquite (Prosopis velutina), cacti such as senita cactus (Lophocereus schotii), chollas (Cylindropuntia spp.) and old man’s head cactus (Mammillaria spp.), which harmonize into a postcard picture of the arid-thorny Sonoran Desert converging towards wetlands of the Gulf of California. The area provides shelter, breeding habitat, and food to a variety of fish and invertebrates, including species of great commercial importance. It’s precisely at this transitional site that western ospreys find the habitat to satisfy their nesting and feeding needs.
During the months of November 2020-May 2021, corresponding to the reproductive season of this species, we detected five active nests and 22 inactive nests, distributed in four different sites between Ejido R. Campodónico and the marine coast. Considering that active nests are those in which we observe behaviors typical of reproduction, including courtship (which consists of calls from the male to the female and vice versa, flights around the nest, food offerings to the female), nest maintenance (carrying branches or fishing nets to the nest), occupation in pairs, copulation, presence of eggs, and presence of fledglings. We have found that the characteristics of the active nests have a larger diameter than the inactive ones (> 4.9 ft) and height (> 14.5 ft), and they differ in the type of materials used for their construction. A more detailed analysis of the in situ environmental variables of the nests will be determined later, as well as the vegetation cover that surrounds each active and inactive nest, and the presence and concentration of environmental pollutants in the western ospreys’ shells and feathers.
To describe the use of active nests, and measure their nesting success, we installed Stealth Cam remote digital cameras in each nest, assuming that each nest was used by the same couple of birds during a reproductive season. This allowed us to find and classify individuals as follows: adults n = 4, reproductive juveniles n = 2 and fledglings, which are individuals of 2.5 months, n = 1. At this site we have also observed that the great blue heron (Ardea herodias) nests in saguaros in areas close to the western osprey nests. The nests of A. herodias are usually smaller in diameter, deeper and shorter than the nests of the western osprey, also building materials are thinner plant remains compared to those used by the western ospreys for their nests, which are usually harder structures such as roots or thicker branches. Western ospreys also considerably use remains of the nets of the different fishing gears in the region. We can observe an apparent perfect adaptation of these raptors to a habitat surrounded by urban and rural communities, fisheries, agriculture and mining.
However, in personal communications with the fishermen, we have learned that years ago more individuals were observed nesting in this region and feeding in the bay throughout the year. They have stated that these birds have a pattern of rapid population changes at the local level, coupled with changes in spatial dispersion close to the places where they are born. This means that a decrease in their numbers at the regional level is easy to see, as they can replace their nesting sites and build nests on power poles, telephone poles, etc. (3). For this reason, each nesting site could vary during each reproductive season if the habitat in some way does not favor the western ospreys’ reproductive success.
Strictly speaking, it is essential for this species conservation to: 1) evaluate how anthropogenic activities may or may not affect their reproduction, 2) evaluate how the habitat in turn suffers a gradual deterioration and becomes contaminated, and 3) take into account the community’s knowledge of their region’s species. These are definite guidelines to improve and build ecological awareness, biological appreciation and respect for landscapes that still remain undisturbed by humans.
Acknowledgements: Dr. Juan Pablo Gallo Reynoso, CIAD-Guaymas and the University of Arizona. In Memoriam EAP.
List of images
- Sighting of a western osprey in December 2020 at the San Jorge Bay Wetlands Ramsar site, the mining tailings from the Caborca municipality gold mine can be seen in the background. Photo: Lucila Armenta Méndez
- Western osprey nest in November 2021 at the La Almeja site of the study area. Photo: Lucila Armenta Méndez
- Remote camera installed in July 2021 in an active nest at the EL Mesteño site of the study area. Photo: Lucila Armenta Méndez
- Adult western osprey in active nest at Boca Ancha site in the study area. The yellow color of the iris is characteristic of an adult specimen. Remote camera photo.
- Adult female western osprey, at the El Mesteño site in the study area. The color of the yellow iris indicates that it’s an adult and the elongated neck that it’s a female. Remote camera photo.
- Pair of juveniles in active nest site at La Clameja. The size and color of the plumage are typical characteristics of one-year-old juveniles. Remote camera photo.
- A recently fledged bird in nest at El Mesteño site, identified by the orange color of the iris, its size and the white coloration of the feathers’ edges. Remote camera photo.
- Russell, S. M., y Monson G. (1998). The birds of Sonora. Tucson, Arizona. University of Arizona Press.
- Rivera-Rodríguez, L. B., y Rodríguez-Estrella, R. (2011). Incidence of organochlorine pesticides and the health condition of nestling ospreys (Pandion haliaetus) at Laguna San Ignacio, a pristine area of Baja California Sur, Mexico. Ecotoxicology, 20(1), 29-38.
- Henny, C. J., D. W. Anderson, A. C. Vera y J.-L. Cartron. (2008). Region-wide trends of nesting Ospreys in northwestern Mexico: A three-decade perspective. J. Raptor Res. 42:229–242