As there are always birds on/around electricity lines… might this happen again?
I thought they had hefty insulators on the cable mounts to stop a short-circuit happening, but perhaps a flock of pigeons bridged the gap somehow?
I know nothing about electricity beyond the switch, but don’t you need an earth connection to be electrocuted?
Yes you do.
So it must have been a very big pigeon to be able to stretch between the live wire beyond the insulator to the body of the pylon which goes to earth. 
No. An Earth is unnecessary for electrocution . . .. any connection between live and neutral puts the full whack of current ..through the load and an earth leakage trip will never detect it .. . An Earth is an additional protection in the case that the person touches the Live and has not as yet touched the Neutral . . It also protects the person if they touch the Neutral before touching the live.. The Earth Leakage detectors are great devices and measures the difference between the currents lowing in both Live and Neutral wires ..which Should be the same . If it detects even a tiny imbalance it opens switches …
You will see flocks of birds happily sitting on on the wires of HT pylons without a care .. that’s because they are not touching any two wires . It would only be at the connection to a house that the wires would ever be close enough for a bird to bridge a gap
Dan beat me to it - no, you don’t.
It can leap within a certain distance from the wire to something that’s earthed. Can even be just if a bush or tree grows close to it and bird is on wire.