The difference in potential between the feet of a bird is small, but the difference between a power line and the ground is larger.
Work Step by Step
When a bird lands on a wire, the voltage drop between its 2 feet equals the voltage drop across the wire between the contact points. The wire has a small resistance, so this voltage drop is small. When a ladder connects the ground and a power line, there is a large potential difference between its two ends, from the high-voltage wire to ground (zero voltage). This large potential difference could result in a large current through the ladder.