Katy Perry knows how to make her mom feel special on her birthday -- get an entire concert audience to sing to her live on a ridiculously huge phone.
Katy -- or "Feather" as her mother, Mary, calls her -- took some stage time during her show in Tampa Friday night to call up Mom for her 70th birthday ... and it was a hit.
Mary then gave the crowd of 10,000 something to look forward to when they get to her age.
Sadly, Katy's mom's bday turned out to be not so great ... the singer says she called her again Saturday, only to find out she was being evacuated from her home due to the California wildfires.
Woke up to wish my mom a happy 70th birthday today to find my brother evacuating them from the insane #ThomasFire in Santa Barbara County that has kicked up once again... praying for the firefighters & first responders that are fighting it back as best they can. Love you SB 💔
Katy's family joins the list of celebs who've been affected by the Thomas Fire ... now the third largest in CA's state history.