In case the pattern calls for something other than an empty nest where an empty nest is present, the process suspends until some bird should place something on the nest (usually as a result of the actions of other processes). A nest with something on it matches the pattern as though the nest were not there, just the (top) something.
If a pattern requires something other than an empty nest, the process pauses until a bird places something on the nest (usually due to actions of other processes). A nest with something on it matches the pattern as if the nest were not there, just the (top) item.