Physiological ecology of migratory birds during the stopover periods
迁徙鸟类中途停歇期的生理生态学研究

Most avian migration is consisted of migratory flights and stopover.Fuel is consumed during the former and deposited during the later period.Avian migrants are adapted to the frequent alternations between energy consumption and deposition.This paper reviewed the studies of the energetic ecology of migratory birds during stopover periods focusing on the rate of energy deposition,patterns of body mass change and phenotypic plasticity of digestive organs.Future research directions of the physiological ecology of stopover migrants were also discussed.Anticipating energetic demanding,avian migrants deposit fat stores before migration.However,they often have to periodically replenish depleted energy stores during stopovers.Fat is the main source of migration energy of avian migrants although more studies recently documented the use of protein during migration.Fuel deposition rate at the stopover sites affects the speed and the probability of a successful migration.Migrants can increase the fuel depositions through increasing foraging time,feeding rate and possibly assimilation rate.Some birds can decrease the fuel consumption rate during stopover through hibernation.Affected by the food resources,inter-and intra-species competition,predation risk and weather conditions,fuel deposition rate varies among stopover sites.Studies have suggested that the availability of food resources is the key factor which determines the fuel deposition rate.Molting,an energetically expensive event,decrease the fuel deposition rate of some species,especially in fall.The pattern of body mass change of stopover migrants is organ and time dependent.The muscle mass of flight organs increases over stopover period and reaches maximum just before migrants depart the stopover sites;the mass of digestive organs increases during the early period of stopover and decreases before migrants depart the stopover sites.The mass of digestive organs just before the departures is similar to that when the birds arrive at the stopover sites.This pattern indicates that birds are adapted to the variations of functional requirements of different body systems between migratory fly and fuel deposition periods and are optimized through special regulation mechanisms.However,this pattern has not been observed in migrants during short-distance migration.During stopover,the size and mass of digestive organs increase with the increase of food intake.However,the activity of digestive enzyme and the rate of assimilation do not change obviously.Many birds change their food choice from carnivory to phytophagy or omnivory,which is coupled with the increase of digestive organs,activity of digestive enzyme,and in some cases,the assimilation rate.However,the phenotypic plasticity of digestive organs is limited,fasting during a long-distance migration can result in the reduction of the size and function of digestive organs.Energetic ecology of stopover bird migrants still need to further address:(1) body compositi