Introduced species to oceanic islands can cause tremendous declines and ext
inctions of native avifauna. On Langara Island, British Columbia, Canada, t
he burrow-nesting ancient murrelet (Synthliboramphus antiquus) has declined
from an estimated original population of 200,000 pairs to 14,600 pairs in
1993. Previously causes of this decline were unknown, but the introduction
of Norway rats (Rattus norvegicus) has been implicated as a major factor an
d has resulted in a recent rat eradication program. A major obstacle in the
investigation of the effects of rats on seabirds, here and elsewhere, has
been the inability to accurately assess the importance of seabirds to the d
iets of rats that also consume a variety of plants and invertebrates. We us
ed stable-carbon (delta(13)C), nitrogen (delta(15)N), and sulphur (delta(34
)S) isotope analyses of muscle and liver tissues of rats and prey organisms
from 3 regions of Langara Island to evaluate evidence for marine foods, in
cluding seabirds, in the diets of rats. Rats were segregated into 3 isotopi
c groups corresponding to upland, littoral, and seabird nesting areas on th
e island. We interpret these groups to represent individuals consuming pred
ominantly C-3 terrestrial foods ((x) over bar delta(15)N = 5.4 parts per th
ousand; (x) over bar delta(13)C = -24.9 parts per thousand; n = 24), intert
idal invertebrates ((x) over bar = delta(15)N = 8.9 parts per thousand; (x)
over bar delta(13)C = -14.3 parts per thousand; n = 21), and ancient murre
let adults, chicks, or eggs ((x) over bar egg delta(15)N = 13.2 parts per t
housand; (x) over bar C-13 = -17.6 parts per thousand; n = 8). We found str
ong correlations between liver and muscle isotope values for both C-13 and
N-15, suggesting that dietary preferences within individuals remained relat
ively constant. Stable-sulphur isotope Values of rat liver were less useful
in segregating marine and terrestrial dietary inputs, possibly because sou
rces of sulphur to the terrestrial food web were of marine origin (pooled (
x) over bar delta(34)S value = 17.8 parts per thousand; n = 20). Our measur
ement of liver and muscle tissues gave dietary estimates based on relativel
y short- and long-term integrations and revealed the 3 groups of rats remai
ned isotopically segregated at least over the 2-month period of ancient mur
relet breeding on Langara Island. Our results have important ramifications
for dietary investigations of introduced fauna and their effects on native
seabirds on oceanic islands.