Heterosis: Hybrid Vigor--is a cross between two disparate parents (either distinct species, or separate populations), where the resulting off-spring is superior (physically, genetically, reproductively, etc.) than either of the parents.
Audubon's Warbler=Yellow-rumped Warbler X Black-fronted Warbler.
Brelsford, A., B. Mila' and D.E. Irwin. 2011. Hybrid origin's of Audubon's Warbler. Molecular Ecology. 20:2380-2389.