Other folks have essentially given the answer, but maybe another few words could help. Imagine a line of avian evolution coming off the therapod dinosaurs (the consensus view of ancestry these days). Then branches will emerge from time to time, leading to various extinct or modern orders; systematists (taxonomists) try to figure out the sequence in which these branches emerged, and they list the orders in that sequence. But science is a never-ending quest for reality, and, as Mike Patterson says, expect revisions, because that's what taxonomists do as more evidence emerges. Some extinct critters like Hesperornis and Ichthyornis emerged early, and didn't survive, but their orders are listed early. The big flightless birds (emus, ostriches, rheas) come early on the checklists of places that are lucky enough to have them. We used to think that loons and grebes were among the earliest-evolved birds that we have in our area, but then other evidence placed the anseriforms (waterfowl) and galliforms (chickenlike) earlier, so they're now being placed earlier in the checklists and some field guides. I haven't heard anything about shorebirds -- is there now some new evidence about them? Anyway, that's the general story.

Burt Guttman
The Evergreen State College
Olympia, WA 98505 guttmanb at evergreen.edu
Home: 7334 Holmes Island Road S. E., Olympia, 98503