This week on Flightless Bird, David Farrier tries to understand why his entire street smells like semen. He finds himself in the world of the Callery Pear Tree - also known as the Linden Tree or the Bradford - AKA “the cum tree”. David talks with Theresa Culley, a professor at the University of Cincinnati - who knows a lot about this particular foul-smelling tree. The plant biologist teaches David why the trees smell like that, before diving into an analysis of how American these trees really are. David is shocked to find out one of these trees survived 9/11, and is now found at the 9/11 memorial site. He also discovers the tree - once the darling of US landscapers and gardeners - is now considered an invasive species. With this in mind, David visits LA’s Natural History Museum to meet herpetologist Greg Pauly, who educates David about some of America’s other invasive species, including one particularly loud and annoying frog.