This is extremely rare: a leucistic water rail. Leucism is a form of albinism where a bird is partly white. The mutants don't usually survive long because they are so visible to predators. Leuco here survives by hiding in the reeds and avoiding open water. He appeared in a flash and was gone, but the 5DIII with the 300mm f/2.8 + 2xTC was quickly swung into action and locked on. This is a 100% crop, with the reeds meant to look like his cage that protects him.
5DS R, 7D II, EOS-M, Powershot G3 X, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22