What is the world's smallest shearwater?

Hint #1: It is smaller than the Audubon's Shearwater.

Hint #2: It is littler than the Little Shearwater.

Hint #3: It has been found in the United States.

Hint #4: It was recently described to science.

The world's smallest shearwater is Bryan's Shearwater (Puffinus bryani).

If you haven't heard the story of the Bryan's Shearwater, it bears repeating.

A small shearwater was discovered and collected in 1963 from a nest burrow in a Bonin Petrel colony on Midway Atoll (a territory of the US, midway between Asia and North America, and 1/3 of the way between Honolulu and Tokyo). It was originally identified as the first record for Little Shearwater in the Hawaiian Islands.

Seabirder Peter Pyle recently re-analyzed the specimen trying to determine what subspecies it may be. Instead, he discovered that it was smaller than any Little Shearwater, but colored more like the larger Audubon's Shearwater. Likewise, DNA analysis found that it was closely related to these other shearwater groups, but didn't fit neatly into any one of them. Thus, in 2011 Pyle et al. proposed a new species.

That is interesting in itself. And in over 40 years no other Bryan's Shearwaters were found on well-studied Midway, or anywhere else... until 1997.

During subsequent years, a total of 6 small shearwaters were captured on various islands in the Ogasawara island chain. Just a few weeks after Pyle published his paper naming the new species from Midway, DNA analysis revealed that these Japanese birds were also Bryan's Shearwaters! Japanese scientists believe there may be several hundred birds on these islands. They remained undiscovered so long because these shearwaters evidently breed in winter when seas are unfavorable for visiting these small islands.