The data sheet says:-
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed,
resulting in 1.0MHz system clock. The startup time is set to maximum and time-out
period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that
all users can make their desired clock source setting using any available programming interface