Bird was said to be close to a fellow taxi driver he killed in Duke Street, Whitehaven, as he started his rampage at about 10.30am.
Glenda Pears, boss of L&G Taxis, said: "We just don't know what's happened. They used to stand together having a craic on the rank.
"He was friends with everybody and used to stand and joke on Duke Street." One Whitehaven cab driver said he understood a total of three taxi drivers had been shot, two fatally.
A taxi driver went on a shotgun rampage today, killing 12 people and wounding at least 25 before turning his weapon on himself.
Divorced father-of-two Derrick Bird, 52, shot several colleagues in Whitehaven in Cumbria, then drove his cab through the countryside of the Lake District blasting other victims apparently at random.
Following a frantic manhunt, police found his body 20 miles away in woods near the hamlet of Boot three-and-a-half hours later.
Cumbria Police said there were 30 separate crime scenes.
"I don't know what caused it, but something must have happened last night. I don't know what time of night it was." He said he knew one of the victims, describing him as "a real character".
The taxi driver, who had known Bird for 10 years, added: "He was quite a friendly person. It's just a tragic thing." One witness said the gunman drove up alongside him at traffic lights by Whitehaven police station in Duke Street.
Alan Hannah, 68, of Great Broughton, told the Whitehaven News: "A car pulled up on the left, stopping at the lights.
Witnesses spoke of their terror as Bird drove through Whitehaven with the shotgun hanging out of the window of his taxi.
Police said that, after leaving the town, the gunman travelled south along the coast, shooting people in Gosforth, Seascale and Egremont before turning inland and ending up in Boot. Rod Davies, landlord of Gosforth Hall Inn, said the close-knit Cumbrian communities were shaken by the tragedies.
"There's a lot of very strong feeling of absolute shock.
There's a lot of fear - a lot of people are expecting to hear names of people they know," he said.