Environmental Mechanisms Underlying The Seasonal Migration Phenology Of East Asian Waterfowl

Migration phenology plays a critical role in shaping bird life histories.Most previous studies have focused on the timing of spring migration and the arrival of birds at breeding grounds along the European and American flyways;knowledge gaps lie in the mechanisms underlying seasonal migration along the Asian flyways.This study focuses on environmental cues and decision rules that waterfowl take in seasonal migration phenology along the East Asian-Australasian Flyway.Firstly,using linear regression models,this study investigates how local temperature variation and EI Ni?o Southern Oscillation(ENSO)influences the arrival and departure timing of nine waterbird species breeding in Mongolia or Siberia and overwintering in Poyang,China from 2002 to 2013.Birds mainly arrive at Poyang in October and depart for their breeding sites in March.Out of the nine species,six show a strong negative relationship between departure time and overwintering temperature in Poyang.Departure dates also show a negative association with overwintering ENSO and March ENSO for two species.Both local and large-scale climate indices show no influence on the arrival timing of waterbirds.We suggest that birds react to the annual variation of overwintering temperature: an earlier departure of waterbirds is facilitated by a warmer overwintering period and vice versa.The long-term accumulated temperature effect is more pronounced than ENSO and the short-term local temperature effect.Secondly,frost is an indicator of cold weather,food scarcity,and water unavailability,but how frost drives the autumn departure of migratory birds has not yet been quantified.In this study we propose the ‘frost wave hypothesis',which posits that the autumn departure of waterfowl is driven by a successive wave of the onset of frost.Using bird satellite tracking data and generalized linear mixed models,we analyze how the departure probability of two waterfowl species during autumn migration is affected by frost,accumulated temperature,food,snow,ice,short-term weather conditions(i.e.,wind,temperature and precipitation),remaining migration distances,relative stopover duration,and flight distances between stopover sites.We find that bird autumn departure probability sharply increases after the first frost spell when the accumulated temperature reaches 0 °C,facilitated by surface meridional wind and longer remaining migration distances.We underline the dominant effect of frost on autumn departure,as birds tend to leave even under head wind if the time lag since the onset of frost is large.Time constraints that trigger southward departure are likely to be stronger when migrating birds are still far from their wintering site.By riding the frost wave,birds manage to maximize stopover site utilization while escaping harsh environmental conditions.Thirdly,food resource emergence differs between natural habitats and agricultural lands for waterfowl species.However,studies investigating resource tracking during spring migration hardly considered the responses to the phenology of food availability at different habitat types.Using bird satellite tracking data and resource selection models,we analyze how the selection probability on the arrival for four waterfowl species during spring migration is affected by snow melt,daily minimum temperature,accumulated temperature,and food availability at natural habitats and agricultural lands.We find that East Asian waterfowl take minimum temperature,accumulated temperature and snow as the main cues of northward migration.During their major stopover at Northeast China Plain,migratory waterfowl could explore grains and corns on agricultural lands as main food resources after snow melts.Using this decision rule,they could overtake the snowmelt and the "green wave" of the natural habitat to catch up with the onset of spring at breeding sites.Our findings improve the understanding of annual avian migration,which could help quantify the impact of global climate change on migratory waterfowl and provide implications for better waterfowl habitat management.