In July, entomologist Greg Johnson of the Medicine Lake National Wildlife Refuge in Montana saw flies feeding on blood coming from a pelican's neck. Knowing that pelican chicks had been dying of West Nile virus, he became suspicious.

Now Johnson figures the flies infect the pelicans.

Stable flies look similar to common house flies, but they have a painful bite.

"This (the stable fly scene) was very unusual because stable flies are only reported to feed on domestic livestock, humans and companion animals," Johnson said.

Johnson then collected about 1,300 flies and divided them into 60 groups. Eighteen groups tested positive for West Nile virus.

"These results suggest that stable flies might be involved in amplification and/or transmission of West Nile virus at the pelican colony," Johnson said.

Johnson also noted that not only is this the first reported instance of stable flies feeding on wild birds, but also the first known case of the flies being infected with West Nile virus.