Abstract
Sexual dimorphism in body size among shorebirds is typically linked to reproductive roles,yet such differences may also lead to ecological differences during the remainder of the annual cycle.Here,we investigate whether the smaller males and larger females of Bohai Black-tailed Godwits Limosa limosa bohaii differ in habitat use and rhythmic behaviours as they refuel in coastal China during northward migration.Based on visual observations,increasing abdominal profiles confirmed their refuelling in the Bohai Bay study area.Using GPS telemetry,we found that males predominantly occurred on coastal mudflats and the adjacent saltworks,whereas females used a wider array of habitats,including freshwater and saline wetlands further inland.In late April and early May,females shifted progressively to these inland habitats.Accelerometry measurements show that on the mudflats,both males and females had the highest intensity of movement around high tide.We interpret this as an expression of their restlessness during the hours they were squeezed into small shoreline areas close to intense human activity.In non-coastal habitats,the two sexes showed similar and pronounced diurnal activity rhythms,with reduced movement intensities in the late-night hours before sunrise.The intensities of movement in inland habitats double those on the mudflats,probably reflecting the high foraging activity required to satisfy food demands because of relatively small arthropod prey in rice fields(Chironomid larvae)and saltworks(brine fly larvae),compared to the larger polychaete prey on mudflats.