"The isotopic content of a warbler’s feather can reveal how far north it was when the feather grew - and to a degree, how far uphill and away from the seacoast. An isotope, remember, is a stable form of an element like carbon or hydrogen, with a specific atomic weight. Carbon can be either C12, with six protons and six neutrons, or C13, with an extra neutron. Add a neutron to plain hydrogen and you get deuterium. The ratio of heavy to normal carbon and hydrogen isotopes is related to latitude: the nearer the North Pole, the higher the proportions of deuterium and C13. These elements follow a path from rainfall to plants to plant-eating insects to insect-eating-warblers. When a warbler goes through its summer molt after nesting, the new set of feathers it grows contains a distinctive isotopic signature. "
Link: Full Article from Berkeley Daily Planet
Chestnut-sided Warbler image © 2005 Michael McDowell