Intertidal mudflats and the conservation of migratory shorebirds
Migratory shorebirds rely on intertidal habitats that are increasingly threatened by habitat loss, human disturbance, and climate change. Investigating habitat availability, its influence on shorebird populations and how to prioritise conservation efforts is critical to inform conservation interventions. In this thesis, I integrate remote-sensing, local shorebird population surveys, high-resolution shorebird tracking, citizen science bird count data, and site prioritisation procedures to address key knowledge gaps in shorebird habitat use, population estimation and site prioritisation across tropical Asia. In Chapters 2 and 3, I developed a framework to map mean tidal flat exposure duration per tide cycle using satellite imagery and global tide models, providing the first large-scale assessment of intertidal habitat availability in Bangladesh. My findings revealed substantial variation in tidal flat exposure among sites and between two successive boreal winters for the same sites. Shorebird abundance was positively correlated with the extent of mudflats with intermediate duration of exposure per tide cycle. This approach may allow intertidal foraging areas critical for conservation to be identified more accurately. Building on these habitat assessments, Chapter 4 tackled the challenge of estimating shorebird populations across vast coastal landscapes. By comparing stratified random sampling and regression modelling, I demonstrated that stratified sampling produced more precise estimates of total shorebird populations in coastal Bangladesh, while regression models tended to overestimate numbers, particularly for species with many zero-counts per 5-km grid cell. These refinements improve monitoring accuracy and provide a robust basis for conservation planning. In Chapter 5, I examined shorebird habitat preferences by linking densities of shorebird species to mean tidal flat exposure duration per cycle and prey availability. Shorebirds consistently preferred areas exposed for 4–6 hours per tidal cycle. Macroinvertebrate sampling confirmed that these areas also supported higher prey densities. These findings indicate that prey density may be part of the underlying causal explanation for the relationship of shorebird preference and exposure duration per tide cycle. Shifting from habitat availability to long-term population trends, Chapter 6 examined the hypothesis that the type of habitat used by shorebirds during migration stopovers is associated with interspecific differences in long-term population trends. By integrating 14 years of shorebird count data with tracking results, I found that coastal-obligate species, which principally use intertidal habitats during migration, were declining at a faster rate than those that used inland wetlands as stopovers. This suggests that widespread loss and degradation of intertidal habitats beyond the well-documented Yellow Sea region is driving population declines. My findings highlight the need for a flyway-scale conservation strategy that extends beyond well-known bottleneck sites to include broader intertidal systems, as well as inland wetlands for shorebird species which rely upon them during migration stopovers. The current level of statutory protection of inland wetlands in Asia is fragmentary. Finally, in Chapter 7, I applied a site prioritisation procedure to identify important shorebird sites in Bangladesh and more widely across South Asia, drawing on insights from previous chapters. Using 818,983 shorebird observations and intertidal habitat area, I found that 62% of priority tidal flats remain unprotected in tropical Asia, with major gaps in Indonesia, the Philippines, and Malaysia. By integrating species abundance data with site-specific tidal flat areas, I demonstrated the trade-offs between cost-efficient conservation and ecologically driven site prioritisation. My results emphasise the need to balance conservation investment with ecological significance to secure the long-term protection of migratory shorebirds. Together, these findings provide a comprehensive framework for shorebird conservation, ranging from fine-scale habitat assessments to broad-scale conservation planning. This research advances tidal flat mapping techniques, improves population estimation and highlights the need to expand conservation efforts beyond well-known stopover sites. By integrating ecological research with conservation planning, this thesis offers practical tools for safeguarding shorebirds and their habitats across tropical Asia and beyond.
shorebirds, migration, tidal flat, conservation, population trends, migratory species, protected area, coastal area
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