The yellow-legged gull Larus michahellis is usually considered as an opportunistic species
that depends on food derived from anthropogenic activity, such as garbage and
fishery discards. However, although it has become a problematic species in many
Mediterranean countries, there is still no information about its status in Tunisia. The
aim of this work was to assess the differential use of marine and terrestrial
resources by the yellow-legged gulls breeding in an urban area on Chikly Island.
Dietary reconstructions were performed through the analysis of regurgitates and d13C, d34S
and d15N of feathers of fledglings. Contrary
to most Mediterranean breeding colonies, and to our expectations, the mixing
model showed that yellow-legged gulls
from Chikly are above all marine foragers. Whereas the Lake of Tunis
was the principal source of food in 2005 and 2007, chicks from 2006 were fed
mainly with prey from the Gulf
of Tunis. Although the Gulf is located further from
the breeding colony and has less fishing activity than the Lake,
our study demonstrated that it is used as an alternative foraging habitat. The
Bayesian mixing model approach proved to be a useful tool for evaluating temporal
variations in the feeding ecology of the colony, which is relevant information
in the management of a wild species. This study also demonstrated the
importance of isotopic variability among years for inferring diet diversity and
food availability for the colony, thereby allowing demographic forecasts when
trophic resources vary in abundance or the foraging habitat is disturbed.
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