This is a whooping crane. I doubt you have seen one in the wild although there are plenty of grey cranes around.
This is what Wiki has to say about the whooper.
The whooping crane (Grus americana), the tallest North American bird, is an endangered crane species named for its whooping sound. In 2003, there were about 153 pairs of whooping cranes. Along with the sandhill crane, it is one of only two crane species found in North America. The whooping crane’s lifespan is estimated to be 22 to 24 years in the wild. After being pushed to the brink of extinction by unregulated hunting and loss of habitat to just 21 wild and two captive whooping cranes by 1941, conservation efforts have led to a limited recovery. As of February 2015, the total population was 603 including 161 captive birds.
Now here is a whopper of a crane; it is the tallest in London, more than twice as high as Nelson’s column. It is on the site of the old Earls Court exhibition centre. The normal crane on the left looks like a matchstick.
It was assembled earlier this year and is being used to lift beams that covered the District line. These beams weigh between 80 and 1,500 tonnes. Apparently 118 London buses weigh 1,500 tonnes but are they empty or full? There are going to be 8,000 homes on the site and five acres of park. New flats on the south side of the river in Battersea and “Embassy Gardens” (aka the less salubrious Vauxhall) are flooding the market. Closer to home Fulham Reach is still sprouting blocks of flats. Will there be anyone to buy in Earls Court on a site between the Warwick and North End roads?