Can a bird, previously at earth potential, get electrocuted by landing on a powerline at high-enough voltage due to the initial "equalization charge"?

Assuming the bird still is at earth potential when entering in contact with the wire (say, it jumped right on it from the pole).

There are lots of unknowns in this problem but let's try to fill some gaps with data we kind of know in humans. So until an EE stackexchanger who is an ornithologist shows up with interesting data, let's assume humans can fly and like to chill out hanging from a high voltage cable.

All objects and living things have an equivalent electrical capacity. The Human Body Model is a convention which dictates humans are equivalent on that aspect to a 100pF capacitor (let's assume it doesn't reduce much from the ground to 23meters high, and call it a worst case scenario). Now, let's assume the contact resistance between the cable and wherever the geometric center of that capacitor is, is 3000Ohm - taken from the "Hand holding wire" case of the table in another thread - divided by two for a two hands contact. Then the total duration of the equilibrium current, taken as 5 times the time constant of the equivalent RC, is 0.75 microseconds.

Effects of currents through living things depend on the magnitude of the current and the duration. I have never seen any study showing any data below 10ms (e.g. the same study cited above), which is not surprising as apparently the response time of the cardiac tissue is 3ms. For 10ms, the current that generates irreversible effects is 0.5A, and it seems to have settled at that point (little dependent on the duration), certainly down to 3ms. Let's assume that past that point, the cardiac tissue behaves like an ineffective first order system, attenuating 20dB/decade. The required current for similar effects would be 20*4.25=90dB higher, or 15811A. For a contact resistance of 1500Ohms as used above, it means the voltage of the cable needs to be 23GV!

Burns solely depend on the energy transferred, so theoretically a high voltage could burn for such a small time. But how high? Well, "Electrical injuries: engineering, medical, and legal aspects", page 72, states:

The estimated lowest current that can produce noticeable first or second degree burns in a small area of the skin is 100A for 1s

Edit: Note that 100A is quite high, it is unclear how the author defines "first degree burns on small area of skin", but I would guess it would be for an area bigger than an inch, burning all epidermis and some of the dermis cells such that they peel away.

So for 750nanoseconds, that's 133MA required! If we use again the 1500Ohms resistance from above, that means the wire would need to be at 199GV, which is insane. Chances are there will be other nasty effects before those burns appear, but neither 23GV nor 199GV sound likely in the near future. Side note, as J... raised in the comments, a 23GV cable would spontaneously arc with anything at Earth potential within 7.6km and therefore would require an incredible amount of isolation.

As if it wasn't enough, you may have noticed that the above assume the maximum current is applied for the entire duration of the equilibrium current whereas in fact it is a decaying exponential... The average current over this duration is in fact 0.2 times the maximum, so these values should really be 115GV and 995GV!

Warning: This does not mean it is safe to jump on and hang from high voltage lines, this is a quick analysis with rough data estimates and modelling and shall not be considered a justification for your actions.

I mostly agree with Andy Aka explanation. I'll give a more detailed theory though (of course I might be overlooking something).

A body doesn't have a capacitance by itself, as it always needs the "second plate" of the capacitor. Humans relative to ground will have a given capacitance when standing (insulated) over then ground, and a different capacitance when flying (if able to) because then ground is farther.

A simple model of the bird could be the one in the next diagram:

^{simulate this circuit – Schematic created using CircuitLab}

As the bird approaches the line C1 will increase and C2 will decrease. This is a capacitor divider and the potential of the bird will approach the High Voltage (HV) line one.

Let's assume, just to give some quick numbers, that C1 is 100 times C2 just before the bird's feets touch the line, the difference of potential between the bird and the HV line will then only be 1% of HV. Finally the bird's feets touches the line: C1 is "shorted" and the only capacitance to fill would be C2 (capacitance between bird and ground, which is very small as ground is far). Because body potential is already at 99% of HV, and it's capacitance to ground is very small, the current through the bird would be really small.

NOTE: My understanding of what happens when a bird flies from an earth object to a powerline (please correct me if I'm wrong) is that - upon contacting the wire - its electric potential changes from earth-potential to the powerline's potential

Here lies the crux of the matter. As the bird leaves the ground heading in the direction of the wire, it acquires a gradual change in potential. This is not an instantaneous change because if it were, the bird would experience a current jolt at that instant it landed.

So, no, it doesn't happen instantaneously and, bigger wire voltages = larger distance therefore a longer time period to reach said wire and, without going into the math, the small imperceptible current that the bird experiences will be the same.

Below is a picture of the way the voltage level changes with distance between ground and a "hot" wire: -

This is fairly classical electric field analysis. Emanating from the centre (assumed to be a point of high voltage) are black electric field lines. These exit in all directions from the wire and hit "ground" at right angles. If you chose any particular one of these E-field lines and "traveled" along it from the ground level by (say) 10% of its length, you would attain a voltage that is 10% of the hot wire.

If you did this thought experiment for all E-field lines at different percentages of the length you'd be able to plot all the lines of equipotential and that is what the red lines are.

As you should be able to see the potential that a small object can attain rising from ground to the "hot" wire is remarkably linear.