Multiple environmental factors and prey depletion determine declines in abundance and timing of departure in migratory shorebirds in the west coast of India

Abstract

Shorebirds around the world have been undergoing significant declines as well as alterations in patterns of migratory phenology due to major changes in the environment. Many of these changes have been linked with anthropogenic factors such as urbanization, environmental degradation, habitat loss and global climate change. We studied the abundance of 15 migratory shorebird species in a coastal wetland in western India that serves as an important stopover and wintering area by many shorebirds in the Central Asian Flyway. The study site was visited once a week during low tide (from 06:00–12:00 h) for 15 years (2005–2019). Shorebird abundance data was collected following direct count and block count methods. To study the departure of shorebirds we monitored shorebird numbers daily and collected benthic organisms three times a month from the shoreline habitats during low tide. Samples were collected from depths of 15 cm using a metal cylinder from areas that typically had the highest densities of foraging shorebirds. Daily rainfall, relative humidity, temperature of air and water and salinity were measured once a week. Generalized Linear Models GLMs (Gaussian family) was used to determine the variation in shorebird abundance in relation to environmental variables (relative humidity, water salinity, rainfall, water temperature, air temperature), and invertebrate prey abundance. We ran two GLMs with Poisson distribution of error to investigate if departure date varied with environmental variables and the abundance of invertebrate prey. Of the environmental variables, mean air temperature, water temperature and salinity increased significantly during the 15 years while invertebrate prey underwent steep declines. Ten of the 15 species’ declines were predicted by the changes in relative humidity, rainfall, water temperature, air temperature, salinity and invertebrate prey abundance. Departure dates also advanced in several species and were affected by relative humidity, air temperature, water temperature, salinity and invertebrate prey abundance. Our models suggest that salinity and invertebrate prey abundance were driving overall changes in shorebird abundance and departure dates. These two variables were negatively related (invertebrate prey abundance declined with increasing salinity). This was evident throughout the study period, during which salinity increased while invertebrate prey abundance declined. Species declines could be due to poor habitat quality and food availability resulting from environmental degradation. Delays in migration could be linked with inability to find enough high quality food to build up food reserves prior to departure. Better habitat management in key wintering wetlands and stopover sites along the flyway are needed help to restore populations of shorebirds.