capacitive action in transmission lines
It is dangerous.

High voltage power lines usually separated 3m minimum. The air (even humid) it is a good insulator keeping the capacitance in very-very low values. Current it is proportion of capacitance and RMS voltage between conductors

I=2πfVrmsC 

In case of a medium size bird –a sphere of 20cm and consequently capacitance of around 10pF placed between the lines/capacitor-, the current will be close to 1mA. 

For humans (sphere 1m) and for 400kV the current may be reach the dangerous 10mA. In 100kV lines the hazard risk is much less. You can search the relevant safety standards dealing on how to aproache HVl ines autorized and non-authorized people.

All above considered a capacitor with the second shell at 0 potential. However HV power lines it is delta source i.e without any common conductor.