No supply if the arduino mega is not connected

Hello there,

I have the XBEE Carrier(1) which I use it to charge a battery with a solar panel.
Once charged, I connected its 3V3 to an attiny85 which should blink just a LED.

But, the attiny85 does not blink the LED even though it have 3.3 V on its VCC.
However, if I add an arduino mega to the same supply (the 3V3 pin of the XBEE Carrier), the attiny85 starts blinking.

Can anyone please explain why the addition of the arduino mega, makes the attiny85 boot ?

(1) http://www.seeedstudio.com/depot/stem-xbee-carrier-p-905.html?cPath=132_134

Have you tried measuring the voltages with a multimeter?

(very radical, I know...)

Yes it's 3.3V .