Watching an animal eat is difficult and produces a biased record, since observation favours daylight, open habitat and tolerant individuals. Isotope analysis reconstructs diet from tissue instead, and covers periods no observer was present for.
What isotopes are measuring
Elements such as carbon and nitrogen occur in forms of slightly different mass, and biological processes handle those forms at marginally different rates.
Those small differences accumulate predictably as material passes from soil to plant to herbivore to predator, so tissue carries a chemical record of the pathway that built it.
Analysis measures the ratio of heavy to light forms in a sample and compares it against local baseline values, since the starting point differs from place to place.
Carbon tells you where, nitrogen tells you how high
Carbon ratios distinguish broad plant types at the base of the food chain, separating tropical grasses from trees and shrubs, and marine sources from freshwater and terrestrial ones.
Nitrogen ratios rise at each step up the food chain, because the lighter form is preferentially excreted. Tissue therefore indicates roughly which trophic level an animal feeds at.
Together the two axes place an individual within a food web. A carnivore eating marine fish and one eating woodland rodents are clearly separable without either being seen.
Different tissues cover different periods
Tissues turn over at different rates, so the choice of sample determines the time window described.
Blood plasma reflects recent weeks, muscle several months, and bone collagen years. Hair, feathers and whiskers stop exchanging material once grown, so they preserve conditions at the moment of growth.
Sampling successive segments along a whisker or feather therefore yields a sequence rather than an average, which is how seasonal diet shifts and migration timing are reconstructed.
Reconstructing movement
Hydrogen and oxygen ratios in rainfall vary geographically in patterns tied to latitude, elevation and distance from the coast, and those patterns pass into local plants and water.
An animal growing a feather in one region carries that region's signature into another, which lets researchers estimate where a migratory bird moulted.
The resolution is regional rather than precise, so the method suits questions about which broad area a population came from rather than about individual routes.
Where the method runs into limits
Interpretation depends on knowing the local baseline, and baselines shift with agriculture, sewage inputs and changing rainfall, so historic samples need care.
Foods with similar isotopic values cannot be separated, which means the technique defines the type of diet rather than naming particular prey species.
It is consequently used alongside other evidence, with genetic analysis of droppings identifying species and isotopes supplying the proportions and the timeframe.