Declining wintering shorebird populations at a temperate estuary in California: A 30-year perspective

Worldwide, shorebird populations are declining. Our objectives were to examine abundance trends of shorebirds regularly wintering at Tomales Bay, Marin County, California, accounting for the local effects of rainfall, raptors, and the restoration of part of the bay to tidal wetlands. From November 1989 to February 2019, we conducted 177 comprehensive winter shorebird surveys of Tomales Bay; we averaged 5.7 ± 0.9 (mean ± SD) winter surveys per year. In 30 yr, we counted 1,215,821 shorebirds of 31 species. We used generalized linear models and multi-model inference to evaluate trends in shorebird abundance while accounting for local sources of variation. We conducted separate analyses for 14 species seen in at least 20 of the 30 yr of monitoring and for all shorebird species combined. During the study, the abundance of all species combined declined 66% (52% in the North Bay and 81% in the South Bay) with the most rapid decline in the first 10 yr of monitoring. Of 13 species for which year was in the top model, 10 species decreased in abundance and 3 species increased. Dunlin and Western Sandpiper accounted for the greatest losses in total numbers. The best-supported models to estimate trends in shorebirds included predictors for year and North Bay vs. South Bay. Of the local variables we considered, rainfall was included in 10 of the 15 best-supported models (including all species combined), negatively affecting the numbers of all species except Willets. The wetland restoration project was included in 5 top models, with a short-term positive impact. Raptor abundance was included in 3 top models with mixed results. Our results show that effective conservation and management of local shorebird populations must be linked with regional/global efforts if we are to reverse negative shorebird trends.

Flow Features of the Near Wake of the Australian Boobook Owl (Ninox boobook) During Flapping Flight Suggest an Aerodynamic Mechanism of Sound Suppression for Stealthy Flight

Synopsis The mechanisms associated with the ability of owls to fly silently have been the subject of scientific interest for many decades and may be relevant to bio-inspired design to reduce noise of flapping and non-flapping flying devices. Here, we characterize the near wake dynamics and the associated flow structures produced during flight of the Australian boobook owl (Ninox boobook). Three individual owls were flown at 8 ms−1 in a climatic avian wind tunnel. The velocity field in the wake was sampled at 500 Hz using long-duration high-speed particle image velocimetry (PIV) while the wing kinematics were imaged simultaneously using high speed video. The time series of velocity maps that were acquired over several consecutive wingbeat cycles enabled us to characterize the wake patterns and to associate them with the phases of the wingbeat cycle. We found that the owl wake was dramatically different from other birds measured under the same flow conditions (i.e., western sandpiper, Calidris mauri and European starling, Sturnus vulgaris). The near wake of the owl did not exhibit any apparent shedding of organized vortices. Instead, a more chaotic wake pattern was observed, in which the characteristic scales of vorticity (associated with turbulence) are substantially smaller in comparison to other birds. Estimating the pressure field developed in the wake shows that owls reduce the pressure Hessian (i.e., the pressure distribution) to approximately zero. We hypothesize that owls manipulate the near wake to suppress the aeroacoustic signal by controlling the size of vortices generated in the wake, which are associated with noise reduction through suppression of the pressure field. Understanding how specialized feather structures, wing morphology, or flight kinematics of owls contribute to this effect remains a challenge for additional study.

Use of semi-intensive shrimp farms as alternative foraging areas by migratory shorebird populations in tropical areas

SummaryEvaluating the ability of anthropogenic habitats to serve as surrogates for natural habitats is an increasingly relevant issue in conservation biology. This issue is especially urgent in tropical coastal wetlands that support large concentrations of migratory shorebird populations and are under pressure from development. Here we evaluated the species composition, abundance, and habitat use of Nearctic migratory shorebirds using recently harvested aquaculture ponds during two non-breeding seasons at shrimp farms surrounding Bahía Santa María (BSM), northwestern Mexico. We also estimated shorebird densities at intertidal units in BSM during and after the harvesting season to explore the connectivity with shrimp farms. Over 25,000 individuals of 25 shorebird species used the surveyed farms (∼13% of shrimp-farm development in BSM; 2014–2015: 10 farms, 994 ha; 2015–2016: 8 farms, 924 ha) during the harvest season. The most abundant species were: Western Sandpiper Calidris mauri, Willet Tringa semipalmata, Marbled Godwit Limosa fedoa, dowitchers Limnodromus spp., Black-necked Stilt Himantopus mexicanus and American Avocet Recurvirostra americana. Numbers of birds decreased as the harvest cycle progressed. Most birds (> 70%) were foraging on the ponds, regardless of tidal stage, while numbers increased during high tide for the most abundant species. At surveyed intertidal areas, shorebird densities were overall similar within and between non-breeding seasons. These results indicate that shrimp farms offer ephemeral but consistent foraging habitats used by non-breeding shorebirds, even in vast coastal wetlands offering a high availability of natural intertidal mudflats. Assuming a similar shorebird use in other shrimp ponds not surveyed within BSM, a significant proportion (> 1% of the biogeographic population) of Willet, Marbled Godwit, and Western Sandpiper, as well as imperilled Red Knot Calidris canutus, might use shrimp farms throughout the harvesting season. Before including current semi-intensive shrimp farms within management plans of BSM, further research is mandatory to assess their utility as alternative foraging habitats for shorebird conservation at tropical areas.