NORTH ADAMS — A glorious pair of birds — giant, mythic, lit with white sparkly lights, and intimidating as dragons — now hang in the rafters of Mass MoCA’s massive Building 5 gallery.

They were not supposed to be here. The birds — phoenixes, and the impressive headliner of “Xu Bing: Phoenix,” now up at Mass MoCA — each nearly 100 feet long, and together weighing more than 20 tons, were a site-specific project commissioned by a Chinese real estate developer for a snazzy glass atrium bridging the two towers of Beijing’s Cesar Pelli-designed World Financial Center.