The Northward migration stopover ecology of Bar-tailed Godwits and Great Knots in the Yalu Jiang Estuary, National Nature Reserve, China : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology, Massey University, Palmerston North, New Zealand
Stopover ecology is the scientific study of the behaviour of migrants, the interrelationships among migrants, and between migrants and their environment, at stopping sites. Many shorebird species are long-distance migrants and require high quality stopping sites to rest and refuel during migration. The suitability of a stopping site depends mostly on food availability, level of competition, and predation or disturbance pressure. Events at stopover may not only affect migration performance, but also the subsequent reproduction or survival, and therefore become the limiting factor for the population sizes of migrants. The stopping sites in the Yellow Sea along the East Asian-Australasian Flyway are used extensively by long-distance migratory shorebirds along the flyway, but very little is known about the stopover ecology of these birds. Moreover, the stopping sites within the Yellow Sea region are under serious threats; some of them are being lost before their importance to shorebirds is fully understood.
In this thesis, my aim was to study the stopover ecology of Bar-tailed Godwits Limosa lapponica and Great Knots Calidris tenuirostris at an important stopping site in the northern Yellow Sea, the Yalu Jiang coastal wetland, during northward migration between 2010 and 2012. I estimated the number of these shorebirds transiting and their passage dates using repeated counts incorporated with Thompson’s modelling approach. I described the type, abundance and characteristics of their benthos resources by benthos sampling. I investigated their dietary compositions, foraging patterns and mechanisms of coexistence by behaviour scans, focal bird observations and faecal dropping analysis. Finally, I studied their predation impact on their main bivalve prey, Potamocorbula laevis, using exclosures.
My results indicated that at least 42% of the East Asian-Australasian Flyway’s northward migrating L. l. baueri godwits, 19% of L. l. menzbieri godwits, and 22% of the Great Knots used Yalu Jiang coastal wetland, thereby indicating the importance of the study area to these species. Polychaetes and bivalves numerically dominated the benthic communities, while one bivalve species, P. laevis, constituted more than three quarters of total macrobenthic biomass during the study period. Great Knots, Red Knots Calidris canutus and Far Eastern Oystercatchers Haematopus ostralegus osculans selected mostly the bivalve P. laevis while Bar-tailed Godwits had a broader diet and selected mostly polychaetes, even though most of their intake was of P. laevis. Moreover, the size of P. laevis preferred by godwits and Great Knots overlapped. Their coexistence seems to be enabled by high resource availability rather than niche separation. The different dietary
selections between godwits and knots possibly led to different foraging patterns on the tidal flats. There was clear behavioural evidence that a digestive bottleneck existed in the bivalve-feeding shorebirds. Such digestive constraints of hard-shelled prey, decreased prey quality (amount of energy per dry mass of shell taken), and increased handling and searching time were potential reasons behind a decline in total biomass intake rate in godwits and Great Knots from 2011 to 2012, despite similar numerical and biomass density of their main prey in both years. Finally, there was evidence that predation by shorebirds had a significant impact on the number of P. laevis in one of the years studied.
My thesis showed the importance of Yalu Jiang coastal wetland to Bar-tailed Godwits and Great Knots during northward migration. These two species competed for the same bivalve prey and their coexistence in the years of study was enhanced by ample food resources rather than niche differentiation. The lack of young P. laevis recruitment in the final year of study and the significant predation impact detected indicated a potential decline in food resources after my study. Long-term monitoring will reveal how these species respond to the changes in prey availability. My study provided important scientific information on the numbers of birds using Yalu Jiang coastal wetland, their prey resource availability, their dietary compositions, and behaviours that are crucial for their conservation management in the reserve and potentially in other stopping sites in the Yellow Sea.