Kingfisher back feathers contain melanin, a brown pigment, arranged beneath a spongy nanostructured keratin layer. That layer scatters light coherently, reinforcing blue and green wavelengths while the melanin absorbs the rest. Grinding the feather destroys the structure and the blue vanishes, leaving brown.
The angle dependence is a direct consequence. A bird flying away along a river reads as an electric blue streak down the back, while the same feathers in flat light or from another angle appear muted. The orange underparts, by contrast, are genuine carotenoid pigment and do not shift.
The hunting behaviour has its own optics. Kingfishers must correct for refraction at the water surface and are thought to judge depth during the dive, entering with the nictitating membrane closed. Precisely how the correction is made, and how much is learned rather than innate, is not settled; young birds have a markedly lower strike success rate than adults.