The Italian manufacturer has revealed that it is waiting on the outcome of talks between the organisers of the two series to avert the three conflicts on the 2019/20 calendars published in June before confirming the Envision Virgin FE driver on its GTE Pro class roster.
A Ferrari spokesman told Motorsport.com: "Sam very much wants to continue in FE, so we are still in discussion because of the clashes.
"He missed the Nurburgring WEC race in 2017 [to contest the conflicting New York FE event], but two clashes would be a problem and is likely to result in a change in his contract.
"We have to wait maybe 10 days to understand the calendars before we make a decision."
has been left blank on the entry for the Silverstone series opener on September 1, but he is scheduled to take part in next week's Prologue test in Barcelona.
The spokesman stressed that Bird, who became a factory driver ahead of the 2016 WEC season, is "99 percent sure" to remain with the marque even if he isn't part of the WEC line-up.
He said that the Briton would likely be given a programme incorporating GT3 races and the big enduros should there be no resolution on the calendar conflicts.
#71 AF Corse Ferrari 488 GTE EVO: Davide Rigon, Sam Bird, Miguel Molina
The Bahrain WEC race in December is on the same weekend as a TBC on the FE calendar, which appears to be set for Marrakesh should it be firmed up.
The Sebring 1000 Miles next March clashes with the Chinese FE round at an unconfirmed venue, while the new Seoul FE fixture in May is in conflict with Spa's WEC round.
FIA President Jean Todt said in June that the governing body was looking into the calendar issues, but warned that it didn't have a "magic stick".
Others on the WEC grid potentially affected by date conflicts with FE include Toyota drivers .
The WEC contracts of Lynn and Lopez take priority, while Buemi's Toyota deal means he is free to skip WEC races that don't require three drivers per car.