why does current flow through something that is earthed?

Your name is physics4ever, shouldn't you know this? We did this in GCSE Physics a couple weeks ago and I think it was because when the bird is on a live wire it's at the same voltage as the wire but no current flows into its body. A body is a poor conductor compared to copper wire, and current goes along the path with the least resistance (I think).
But if a bird (or a power line worker) accidentally touches an electrical "ground" while in contact with the high-voltage wire, she completes an electrical circuit. The earth, and anything touching it that can conduct current, is the ground. And I'm guessing the earth route has less resistance? Sorry for the poor explanation.

Current is pushed down a voltage (potential )gradient because voltage is a sort of "electrical pressure" The potential of earth is 0 V and always stays at 0 V because it is too massive to have it's charge changed noticeably by a human power cable. Therefore current will just keep on flowing into it. Earth is basically a "bottomless charge sink". With the bird on the wire now it's at the same potential as the wire so current won't be pushed through it

for example when birds perch on pylon wires ,no current flows through them,but if a mans fishing rod line reaches the wire the current flows through the line to him to the earth? ive never understood why the current flows to the earth. any explanations?

how does the bird gain a potential thats the same as the wire?

how does the bird gain a potential thats the same as the wire?

If you touch a charged object you become charged yourself via electron transfer.

Next week : "so where does that antineutrino come from?"

It's the word 'potential' that holds the key.

By touching the wire, the bird is connected to a source of potential energy carried by the cable.

Voltage is defined as Joules/Coulomb. i.e. a voltage is a measure of the energy transferred per coulomb quantity of charge.

A battery voltage (for example) therefore is a measure of how much energy per unit charge can be supplied. But until there is a conduction path, the energy stored in the battery stays as just a potential energy. Likewise, the bird by virtue of touching the cable, has gained potential energy.

A good analogy is gravitational potential energy: if you sat on a ledge high above ground, you have a lot of gravitational potential energy by means of your height.

That potential energy can only be converted to kinetic energy when you jump off the ledge.