Stable isotope analysis is an ever-changing and ever-growing technique that combines basic ecological concepts with relatively simple chemistry. Before I completely confuse whoever may read this, you may want to check out this site (http://www.uga.edu/~sisbl/stable.html) for a brief introduction to what stable isotope analysis is all about.
The first, and most basic concept that stable isotope analysis draws upon is that everything is interconnected in an ecosystem (and in a larger sense: the entire planet). One may think of a large web in which each strand is connected to adjacent strands. Remove or distort one of these strands, and the structural integrity of the whole web is effected. Moving on.
Every living thing has some atoms in common, i.e. every living thing incorporates carbon and nitrogen atoms into their living tissues. These atoms exist in different forms, called isotopes. What differentiates these forms of the same atom is not their function, but their weight. For example, most carbon has 6 neutrons, 6 protons, and 6 electrons. Since electrons do not contribute to the atomic weight of an atom, this typical carbon molecule can be said to have an atomic weight of the 6 protons plus the 6 neutrons, which sums to 12. But if you were to look up the atomic weight of a carbon atom in the periodic table, you would see that the listed atomic weight was 12.011. This is because carbon can exist as heavier and lighter isotopes. For example, there is one carbon isotope that consists of 7 neutrons, 6 protons, and 6 electrons, giving that atom the atomic weight of 13, but it still retains all the properties of a carbon atom. Therefore we call this isotope C13. The 12.011 atomic weight that you see in the periodic table is a function of the natural abundance of not just the 'normal' carbon with the atomic weight of 12, but also of all the carbon in the environment that exists in heavier (or lighter) isotopes. Whew.
Now, when any living thing (the predator) eats something else (the prey), the isotopes in the prey move up the food chain into that predators living tissue. The interesting thing is that the lighter isotopes are selected to run that predators various metabolic processes, while the heavier isotopes, like C13, remain sequestered in the predators living tissues. If this predator is consumed by something else, then these heavier isotopes bioaccumulate up the food chain. So if C13 concentrations increase as they move higher up in the food chain, then by analyzing this concentration of C13 of various organisms in an ecosystem, someone could see who was on top of the food chain, who was on the bottom, and any organism in between.
Lucky for us, there is a machine that can analyze these heavy isotopes! These measurements of the relative amount of heavy isotopes in an organisms living tissue are called isotopic signatures.
But wait! That's not all! Depending on how the plants photosynthesize in an ecosystem, the concentration of heavy isotopes, thus the isotopic signatures, differentiates. For example, marsh plants photosynthesize differently than most other plants (which is another topic for a another time). Thus all the organisms that use the mash will have a unique 'marsh' isotopic signature while organisms inland will have a different isotopic signature and so forth.
So what does this have to do with shrikes? Well, it may help answer the question on whether or not we have the endangered migratory subspecies L. l. migrans wintering here in Georgia. In Georgia, shrikes live in many habitats including agricultural fields, open pine forests, and even on the coast. If you were to collect a feather from a shrike that was grown in each of these ecosystems, you would presumably get different isotopic signatures for each habitat type. Now feathers are neat because they 'freeze' the isotopic signature of wherever that feather was grown. So if a feather was grown in an inland breeding ground, but then that bird moved and grew more feathers on a coastal wintering ground then those two feather types grown in different habitats should have different isotopic signatures. If you get two distinct isotopic signatures from two different feathers from ONE bird, then that bird must have migrated between its breeding and wintering grounds, i.e. there is a strong chance that this bird may be L. l. migrans!
This is just one application of stable isotope analysis in ecological research, but I figured I have bored you enough already (if you have managed to stay with me for this long).... So here are some pictures of Loggerhead Shrikes from my latest outing around Athens, GA! Special thanks to Vanessa and DeeAnne for helping on that frigid day!
