Isotopes are different forms of the same element that have a different number of neutrons. The isotopes of an element can be used to trace the movement of animals because they are incorporated into the animal's body tissues from the environment.
The isotope that is used to track hawks is strontium-87 (Sr-87). Sr-87 is found in the soil, water, and plants, and it is incorporated into the hawk's body when the hawk eats. The amount of Sr-87 in the hawk's bones and feathers reflects the amount of Sr-87 in the environment where the hawk grew up and lived.
By analyzing the Sr-87 isotopes in a hawk's feathers, scientists can determine the geographic location where the hawk grew up and lived. This information can help scientists to track the movement of hawks and to identify important habitats for these birds.
