You modeled the feet and the head as separate capacitor plates, I was not expecting that, how I imagined it is since current would enter the bird at one point, pass trouigh its body, and exit somewhere when it is in a circuit, since it only has an entry when it sits on the power line the electrons would just be pumped and compressed in the body, like the whole bird would have an excess charge, not just the feet.
I also would not model the bird as two plates. To do that you would need to say that the rest of the bird is a dielectric, but birds conduct reasonably well (chicken conductivity is used reliably in slaughterhouses)
Maybe this schematic is clearer. Capacitor C2 is the air gap between feet and power line before landing. After touchdown, it is just an ohmic contact. The only importance of C2 is that its air gap is the location of the electric arc. My thought was the bird may be more sensitive to a current in the form of an electric arc during landing, than to the same current spread out over a larger contact area after landing. There is no dielectric in the model, only air and conductors.