What is default Clock Frequency for Atmega8
Many AVRs (like ATmega8, ATmega16, ATmega32) have internal RC oscillator. It is calibrated for 1,2,4 or 8 MHz. It has +-3% accuracy. Frequency of oscillator gets affected by VCC and temperature.
(source: http://www.elecrom.com/2007/10/15/configuring-avr-fuse-bits-for-clock-selection/ )