We investigated the relationships between the δdeuterium (δD) and the δ(18)oxygen (δ(18)O) of drinking water and the δD and δ(18)O of blood plasma, red blood cells and feathers in house sparrows (Passer domesticus) fed on diets with identical hydrogen and oxygen isotopic compositions and five isotopically distinct drinking water treatments. We expected and, with only one exception ((18)O in blood plasma), found linear relationships between the δD and δ(18)O values of drinking water and those of bird tissues. The slopes of these relationships, which estimate the percentage contributions of drinking water to the tissue isotopic signatures, were lower than those of previous studies. We found significant differences in the δD and δ(18)O values of feathers, red blood cells and plasma solids. In feathers and red blood cells, δD and δ(18)O values were linearly correlated. Our results have three implications for isotopic field studies: (1) if the isotopic composition of drinking water differs from that of food, its effect on tissue isotope values can confound the assignment of animals to a site of origin; (2) comparisons of the δD and δ(18)O values of different tissues must account for inter-tissue discrimination factors; and (3) δD/δ(18)O linear relationships are probably as prevalent in animal systems as they are in geohydrological systems. These relationships may prove to be useful tools in animal isotopic ecology.