If you want computational power on the scale of the brain, power consumption is a real concern.
The same with with brains. As far as I can tell, transistors are orders of magnitude more energy efficient than synapses. Synapses use tons of slow chemical reactions to send a signal. Transistors just send a few electrons near light speed.
Transistors are not more efficient than synapses. Neurons and transistors in subthreshold transport charge in the same way, through diffusion. Neurons just have a more efficient structure leading to less energy expenditure for an equivalent amount of information processing as the transistor.
If you operate your transistor above threshold as done in all digital circuits, you are orders of magnitude less efficient.
There is no question that birds are more energy efficient than planes in terms of energy expended to move the same mass the same distance.
However, humans would prefer to spend large amounts of energy by burning fuel to cross the Atlantic in 6 hours. It is much more COST efficient, but that is because energy is cheap.
You of course have to consider what you are measuring. Birds have a very low speed, but they are more energy efficient in terms of mass times distance travelled per energy.