Today's mystery bird for you to identify

This reclusive bird closely resembles another similar (much smaller) species that it hybridises with when their ranges overlap, leading some ornithologists to think of them as subspecies instead of full species. Can you name this mystery bird along with its close relative?Mystery Bird photographed at Anahuac National Wildlife Refuge, Anahuac, Texas, USA. [I will identify this bird for you in 48 hours]Image: Joseph Kennedy, 18 November 2010 [velociraptorize].Nikon D200, Kowa 883 telescope with TSN-PZ camera eyepiece 1/320s f/8.0 at 1000.0mm iso400This reclusive bird closely resembles another similar (much smaller) species that it hybridises with when their ranges overlap, leading some ornithologists to think of them as subspecies instead of full species. But this mystery bird differs from its sister species in several important ways. Can you name this mystery bird along with its close relative?Daily Mystery Bird Rules: 1. This is a game, not a testosterone-driven competition designed to crush one's fellow birders/twitchers into the muck of disappointment, despair and humiliation. It is meant to be a learning experience where together we can learn a few things about birds and about the process of identifying them (and maybe about ourselves, too).2. Please name at least one fi...