now, the persistence of hybrids in northern climates apparently may be another evolutionary problem that can be solved, but only rarely: the fonseca paper obtained genetic evidence that the recent colonization of america by "culex" mosquitoes actually involves a strain derived from a rare successful hybridization between "c. pipiens" and "c. molestus". they suggest hybridization may explain why the american form bites both birds and humans (this interpretation is controversial, see letter from spielman "et al." and the response that follows it in "science"). the consequences of this more indiscriminate feeding hit the news in 1999 with the outbreak of human encephalitis in new york, caused by west nile virus. it was the first documented introduction of this virus into the western hemisphere; perhaps because in the longer established populations, the old world northern above-ground "c. pipiens" almost exclusively bites birds, with the human-biting ones being incarcerated below ground.