Sea ducks are making a comeback in San Francisco, but Neotropic birds are in shorter supply, according to the preliminary results of an annual bird count encompassing around 170 square miles of the Bay Area.

More than 100 birders trekked out before dawn Tuesday with their binoculars and documented about 158 species for the annual Golden Gate Audubon Society Christmas Bird Count.

But that number is down from 172 last year, which was already on the low end, said counter Dan Murphy.

After the annual dinner where results are shared, Murphy noticed there weren’t any Neotropic birds, which are from an area that includes South America, the West Indies, Central America and tropical Mexico. Warblers, tanagers and orioles were among rare species that were missing.

“That bothers me as a birder, but not from the point of view of a citizen scientist. It’s too soon to analyze it,” Murphy said.

The annual data that’s been compiled since the 1930s helps scientists analyze the migratory habits of birds and what may be affecting their fluctuations.

Murphy said between 1983 and ’87, birders saw an average of 205 sea ducks around the Bay Area, but then in the most recent five-year span, the average dropped to eight.

But this year, 32 were spotted at Fort Funston and one more in the Bay, and he’s hopeful they’re on their way back.

“We’re just seeing this enormous fluctuation. Whether it’s global warming or pollution or whatever, it’s just too soon to tell,” Murphy said.

The official count could be revealed as soon as mid-January or as late as February, he said.

kkelkar@sfexaminer.com