James Easton was the first to suggest that what Arnold actually saw was a flock of American White Pelicans, the largest birds in North America. The object depicted above does look somewhat bird-like. But many UFOlogists mocked that conclusion, calling it "Pelicanism."
The late Philip J. Klass suggested that Kenneth Arnold probably saw Meteor Fireballs.
An amazingly long and detailed investigation, The Singular Adventure of Mr. Kenneth Arnold (147 pages), by the Scottish UFOlogist Martin Shough - everything you ever wanted to know about the Arnold sighting. He concludes that the objects Arnold described could not have been birds or other prosaic objects, but does not suggest what he thinks they were.
Starting from Shough's investigation, Martin Kottmeyer wrote Joining Shough's Singular Adventure (see page 28), suggesting that Arnold just might have seen Pelicans after all.
Some of Arnold's statements made the objects sound very much like a flock of birds:
In these interviews with reporter Bob Pratt, Arnold gives us good reason to doubt his credibility. He talks about "mystery submarines," says that his phone line has been tapped, that UFOs may be alive, and they seem to be able to read his mind. He says he has spotted UFOs "seven or eight times."