(Las Vegas Review-Journal, 11/7/11) Nevada is "dotted with scores" of 4-inch diameter pipes marking old mining claims -- "perhaps as many as a million of them .... Their yawning mouths point skyward to entice birds that like to nest in burrows and the hollow trunks of trees. The birds go into the pipes but can't get back out because they can't climb the slick interior or spread their wings to fly. Trapped at the bottom, the birds slowly starve to death. 'Basically they're black holes for birds, literally and figuratively .... Once they're inside, it's a one-way trip'."
http://www.lvrj.com/news/mining-claim-markers-blamed-for-thousands-of-bird-deaths-133346378.html