STABLE ISOTOPES
Many chemical elements naturally occur in more than one form. All forms
(both stable and the less stable, radioactive isotopes) have the same number
of protons but differ in the number of neutrons in the nucleus. For example,
both Carbon 12 (^12 C) and Carbon 13 (^13 C) have six protons per nucleus but
differ in the number of neutrons, six and seven respectively. Carbon 12 is
around 100 times more common than Carbon 13 in nature. A 50 kg human
contains about 11.4 kg of Carbon 12 and 140 g of Carbon 13.
Various stable isotopes have proven useful in palaeontology, archaeology
and ecology.^2 One reason for their utility is the fact that the different isotopes,
with their different atomic masses, behave differently as they pass through
natural systems, and become incorporated in animal tissues. For example,
the heavier isotope of nitrogen, Nitrogen 15, tends to be retained by organ-
isms while the lighter ‘regular’ isotope, Nitrogen 14, is excreted. Thus, when
one organism eats another, Nitrogen 15 becomes more concentrated in the
animal doing the eating. As this isotope accumulates further up the food
chain, so an animal’s nitrogen isotopic signature, ascertained via mass spec-
trometry, is an indicator of its diet, be it a top predator, an animal that eats
grazers, or a grazer itself.
Not only do isotopes differ in how they are processed by animals and
plants, they can also show broad geographic patterns. For example there are
changes with latitude in the sea in both the^13 C/^12 C and the^15 N/^14 N ratios. On
land, the ratio of the hydrogen isotopes^2 H/^1 H tends to decrease at higher
latitudes and altitudes (^2 H is deuterium), while there is a complex interaction
between latitude, altitude and rainfall on the ratio of the oxygen isotopes
(^18) O/ (^16) O. Both (^15) N/ (^14) N and (^34) S/ (^32) S (sulphur isotopes) ratios are higher in the
sea than on land. Potentially therefore, the stable isotopes prevailing in the
environment where a bird turns food into body tissues, be it new flesh or new
feathers, can leave an informative signature. This signature enables research-
ers to understand where in the world the new tissues were manufactured.
In the case of seabird studies, stable isotopes have been used to shed
light on diet and roughly where a bird was when new tissues were grown. The
accuracy of that geographical information may be no better than a few thou-
sand kilometres, but it is better than no information at all, especially when it
can be gleaned from a tiny blood or feather sample, all that is required for
successful mass spectrometry. Therefore the technique has the ethical and
scientific merits that it is harmless and can potentially be repeated several
times during a bird’s life.