Physiological challenges of migration

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Migratory birds face a host of physiological challenges during their annual peregrinations including (a) maintaining synchrony with the changing environments encountered as they migrate by relying on environmental cues in conjunction with endogenous rhythms to coordinate patterns of movement and stasis; (b) using fats as their primary fuel to satisfy the very high-metabolic costs of continual flapping flight—this requires more oxygen and lipid transporters and energy production, produces more reactive species, produces less metabolic water, and generates more heat. Overcoming these challenges requires flexible (reversble) modifications to physiology including key components of the endocrine, circulatory, respiratory, antioxidant, and thermoregulatory systems, all of which we have described here. A major theme of this chapter is that the life history of a typical migratory bird includes two migration stages—vernal and autumnal—that occur at separate times of the year under differing environmental conditions, that are regulated by unique neuroendocrine mechanisms, and that present somewhat similar yet distinct physiological challenges. Major gaps remain in our understanding of both the network of specific molecules and regulatory relationships that maintain and adjust homeostasis across the life history stages, and especially how the concentrations of key molecules and the relative strengths of certain regulatory relationships change with the context and the conditions of vernal and autumnal migration. More integrative studies (from molecules to genes to physiology to whole organisms) that are also comparative (multiple systems, multiple tissues within the same individual, migration state vs. nonmigration periods, vernal vs. autumnal migration, multiple species that differ in migration strategy) are needed in order to gain a more complete understanding of how the environment influences migration of birds, how birds overcome the multifaceted physiological challenges of migration, and how individuals will cope with the challenges of climate change.

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Sturkie's Avian Physiology