Host-feeding patterns play a key role in the transmission of vector-borne diseases such as West Nile fever, which involves two kinds of vertebrates, birds and mammals. In this study, we propose a theoretical formulation for mosquito host-feeding patterns using three quantities, as follows: the apparent attractiveness/contact probabilities, the conditional host(-feeding) preferences, and the enzootic versus bridge probabilities. Using results from host-baited trap collections, the quantities deÞned above were assessed for the most abundant mosquito species in the main West Nile virus focus of southern France.Wefound that host availability is important in determining the efÞciency of bridge vectors, and that even ornithophilic mosquitoes like Culex species, classically classiÞed as enzootic vectors, may turn out to be efÞcient bridge vectors in certain contexts of host abundance. Our developed theoretical framework can easily be adapted and applied to other experimental data and other vector-borne diseases.