The geese who flew home for Christmas.

Gray geese of the bar-headed type (Anser indicus synonym: Eulabeia indica) perform trans-Himalayan migratory flight at extreme altitude. The physiological mechanisms include hyperventilation of cold air, increased lung diffusion capacity, proton gradients, high muscular temperature, amino acid mutation in haemoglobin and cytochrome oxidase, re-location of mitochondria, and extreme shifts in the oxy-haemoglobin dissociation curve due to Bohr- and temperature effects. The second annual migration takes place in late autumn, and the bar-headed geese come home in due time before Christmas.