How often have you found a bird's nest in a guardrail? I can't even count the times. However, have you ever found abandoned eggs in one of those nests? You want to be VERY careful handling them! What causes birds to think this would be a good location? Doesn't it get a little too hot for them, the eggs, and their chicks?
When you get here, you will see what happens when a cacher spends too much time looking into the depths of a guardrail. Hopefully this isn't someone you know!