Ecophysiology of avian migration in the face of current global hazards

Abstract

Long-distance migratory birds are often considered extreme athletes, possessing a range of traits that approach the physiological limits of vertebrate design. In addition, their movements must be carefully timed to ensure that they obtain resources of sufficient quantity and quality to satisfy their high-energy needs. Migratory birds may therefore be particularly vulnerable to global change processes that are projected to alter the quality and quantity of resource availability. Because long-distance flight requires high and sustained aerobic capacity, even minor decreases in vitality can have large negative consequences for migrants. In the light of this, we assess how current global change processes may affect the ability of birds to meet the physiological demands of migration, and suggest areas where avian physiologists may help to identify potential hazards. Predicting the consequences of global change scenarios on migrant species requires (i) reconciliation of empirical and theoretical studies of avian flight physiology; (ii) an understanding of the effects of food quality, toxicants and disease on migrant performance; and (iii) mechanistic models that integrate abiotic and biotic factors to predict migratory behaviour. Critically, a multi-dimensional concept of vitality would greatly facilitate evaluation of the impact of various global change processes on the population dynamics of migratory birds.

Figure 1.

Summary of physical and biotic factors that influence the three main physiological attributes of importance to a migrant: vitality (as the overall requirement to avoid death and maximize success, be that of migration or any other stage during the annual cycle), body stores (as prime determinants of flight range and ability to meet nutritional needs) and fuelling rate (as a prime determinant of speed of migration). Each of these attributes is interconnected, and may be affected by global change hazards that result in reductions in habitat size (dark grey); reductions in habitat quality (light grey); and climate-induced changes to phenology and physical conditions while en route (black). The potential links between the various factors and how these affect migration (e.g. habitat loss may lead to increased competitor density, and a subsequent increase in exposure to disease agents or reductions in food quality) are not indicated.