A tiny unborn hummingbird is getting in the way of a big bridge project near San Francisco.

The discovery of a nest and egg in a tree is stalling a $US70 million ($A92 million) upgrade to the Richmond-San Rafael Bridge about 50 km north of San Francisco, officials said on Tuesday.

The species, Anna’s Hummingbird, is protected under the Migratory Bird Treaty Act that forbids the removal of the egg.

The nest – about half the size of a fist – was discovered in a tree set to be removed about a week ago.

Under the protection act, the tree must stay put until the hummingbird baby is gone.

“We’ve dealt with this on all sorts of things on every project we’ve worked with in the Bay Area,” said Metropolitan Transportation Commission spokesman Randy Rentschler.

It could be delay the project by a few weeks but the lag is not expected to increase the cost as crews could complete other work in the meantime, Rentschler said.

But it isn’t the first time a bird frustrated infrastructure projects, cliff swallows built intricate nests beneath an overpass in Petaluma and cormorants held up deconstruction of the old Oakland-San Francisco Bay Bridge, Audubon California spokesman Garrison Frost said.