Plywood outdoors

[Bonehenge (B A R R Y)]

I've seen the same in my back yard. I think the birds need the human smell to weather away.

[Nova]

Birds primarily rely on sight and hearing. Although there are exceptions (turkey vultures, kiwis...) the sense of smell in most birds is not very developed. Human scent most likely would not come into play.

[Bonehenge (B A R R Y)]

Do they have a natural aversion to things that are "new" in the environment?

Maybe that's it.