Here's a rule of thumb: if something is so toxic that birds die when they land on it, it's probably a good idea to put a cover on it.
In Texas yesterday, a federal district court upheld a criminal conviction against the oil refinery giant Citgo Petroleum after migratory birds landed in two open-top refinery tanks and died. Under Environmental Protection Agency (EPA) rules and Texas law such tanks are supposed to be covered. The court found that the company had known birds were landing and dying in their tanks but failed to do anything about it.
Citgo faces some $2 million in fines. A too small price to pay I think for their negligent behavior.
In Canada's tar sands, similar cases of migratory bird deaths have been ruled very differently. Many will remember the story in 2008 when 1600 ducks landed on the massive lakes of tar sands toxic by-products and died.