Arctic spring: The arrival biology of migrant birds

On arrival in the Arctic, migrant birds must adjust their physiology and behavior to unpredictable snow cover, weather, food sources and predator pressure. In other words they must be resistant to environmental perturbations (stress) so that they can migrate to their tundra nesting areas and settle on territories as soon as possible. They can then begin breeding as soon as when environmental conditions become favorable. They do this partly by using micro-habitats such as areas where snow depth is low, and patches of tundra that melt out rapidly (especially near willows Salix sp). Ground temperatures increase dramatically within hours after exposure to sun; and invertebrate activity begins simultaneously. Wind speeds are attenuated almost completely within 10 cm of the ground in willows and tussock tundra. The combination of these conditions provides an ideal refuge, especially for passerine migrants in early spring. However, if conditions worsen, the birds can leave. There are adjustments of the adrenocortical responses to stress because arctic conditions in spring are potentially severe, at least compared with wintering grounds to the south. Secretion of corticosterone in response to acute stress is enhanced at arrival in males, accompanied by a decrease in sensitivity to negative feedback and a change in responsiveness of the adrenal cortex cells to adrenocorticotropin. There is also an increase in levels of corticosterone-binding globulin (CBG) so that the actions of corticosterone are buffered according to the severity of environmental conditions. Regulation at the level of genomic receptors, particularly the low affinity glucocorticosteroid-like receptor for corticosterone in brain and liver, may be important; and non-genomic actions of corticosterone may play a major role too. In other words, the hormone-behavior system associated with arrival biology is highly flexible.