How can birds stand on electrical power lines without being executed?
The true is that some times they do electrify themselves... especially if they try to straddle on two wires at once. But most of the times not.
An electric current is simply flow of electrons. This flow occurs inside a wire, which is typically made of metal. This is due to the fact that metals have very low resistivity for the electron's motion (another term is "conductor"). However, in order for the current to flow at the first place, it needs to have a "reason" to flow - namely, an electric potential. This is very similar to a situation where one has a bunch of bowling balls up a mountain; give them a path, and the balls will naturally roll down the mountain. At the top of the mountain they have high potential, and they will travel down once a suitable path becomes available for them. Returning to electrons on a wire, the wire provides the necessary path (of least resistance), while the voltage (supplied by the power plant) provides the needed potential.
When a bird sits on a wire and the electrons reach the part of the wire where the bird is sitting, the electrons have two options. They could go through the bird's feet and encounter a large amount of resistance or they could go through the metal. All metals are conductors which means that electrons flow through them easily. Because it is easier to travel through the metal instead, the electrons don't go through the bird's feet, so the bird stays safe.
However, this situation changes if the bird stretches out a wing or a leg and touches a second wire, especially one with a different electrical potential. In such an unfortunate event for the bird, it will open a new path for the electrons,right through the bird's body, which will be electrified.
There are other perils for the birds. The wood pole supporting the wires is buried deep in the ground, so it would also be dangerous for a bird to sit on the pole and touch a wire. This is the problem that people encounter if they touch live wires - since we are almost always in contact with the ground. Our bodies turn out to be excellent conductors of electricity, and the electrical current will happily use them to complete a closed path to flow from high potential (the wire) to low potential (the ground). ZAP!