Seabirds

http://en.wikipedia.org/wiki/Seabird
"There exists no single definition of which groups, families, and species are seabirds, and most definitions are in some way arbitrary. In the words of two seabird scientists, "The one common characteristic that all seabirds share is that they feed in saltwater; but, as seems to be true with any statement in biology, some do not."^[2] However, by convention all of the Sphenisciformes and Procellariiformes, all of the Pelecaniformes except the darters, and some of the Charadriiformes (the skuas, gulls, terns, auks and skimmers) are classified as seabirds. The phalaropes are usually included as well, since although they are waders ("shorebirds" in North America), two of the three species are oceanic for nine months of the year, crossing the equator to feed pelagically.
Loons and grebes, which nest on lakes but winter at sea, are usually categorized as water birds, not seabirds. Although there are a number of sea ducks in the family Anatidae which are truly marine in the winter, by convention they are usually excluded from the seabird grouping. Many waders (or shorebirds) and herons are also highly marine, living on the sea's edge (coast), but are also not treated as seabirds."

More detail on the major branches

http://www.owlpages.info/downloads/A_Phylogenomic_Study_of_Birds_Reveals_Their_Evolutionary_History.pdf
"The largest clade in Neoaves was a well supported
land bird clade (green, node F, Fig. 2)
(3) that contained the Passeriformes (perching
birds, representing more than half of all avian
species), which is allied with several morphologically
diverse orders. These included Piciformes
(woodpeckers and allies), Falconiformes (hawks
and falcons), Strigiformes (owls), Coraciiformes
(kingfishers, hornbills, rollers, and allies), Psittaciformes
(parrots), Coliiformes (mousebirds), and
Trogoniformes (trogons). One of the most unexpected
findings was the sister relationship
between Passeriformes and Psittaciformes (node
A, Fig. 2), with Falconidae (falcons) sister to this
clade. This relationship varied slightly among
analyses and gene-jackknifing (Fig. 1), yet the
close relationship between passerines with parrots
and/or falcons appeared consistently.
Sister to the land birds is the Charadriiformes
(shorebirds, gulls, and alcids; yellow, node G,
Fig. 2). This grouping seems to be driven primarily
by the b-fibrinogen gene (FGB), because
it was present in analyses of only this gene and
disappeared when the gene was removed through
jackknifing (Fig. 1). Regardless of the exact
placement of the Charadriiformes in our analyses,
we consistently support that this order is not
basal within Neoaves (24) and thus refute the
hypothesis that transitional shorebirds gave rise
to allmodern birds (7). Our phylogeny revealed a
highly supported water bird clade (blue, node H,
Fig. 2) (3, 14), including members of the
Pelecaniformes (totipalmate birds), Ciconiiformes
(storks and allies), Procellariiformes (tubenosed
birds), Sphenisciformes (penguins), and
Gaviiformes (loons)."

But notice that they have lumped the shorebirds, gulls, and alcids together.

Strong Support

Here is strong support for what I am proposing:
http://www.owlpages.info/downloads/A_Phylogenomic_Study_of_Birds_Reveals_Their_Evolutionary_History.pdf

"Modern birds occupy a wide diversity of
niches and exhibit a variety of behaviors. The
broad structure of our phylogeny suggested diversification
along general ecological divisions,
such as water birds, shorebirds, and land birds.
However, adaptations to these environments clearly
arose multiple times, because many aquatic birds were not part of the water bird clade (e.g.,
tropicbirds, flamingos, and grebes) and terrestrial
birds were found outside of the land bird clade
(e.g., turacos, doves, sandgrouse, and cuckoos).
Our phylogeny also indicated several distinctive
niches, such as nocturnal (owls, nightjars, and
allies), raptorial (falcons, hawks, eagles, New
World vultures, seriema, and owls), or pelagic
(tubenosed birds, frigatebirds, and tropicbirds)
lifestyles, have evolved multiple times."

 

Taking this a little further, the birds listed in green are from the landbird line.

The birds listed in gold are in two different lines. Some (Charadriiformes/Charadrii) are in the wader line and the others (Charadriiformes/Lari) are in the seabird line.

The birds in blue come from the seabird line.

The birds in red are in two different lines. Some are in the waterbird line and the others are in the landfowl line.

Petrels

The pattern of development is interesting.
http://en.wikipedia.org/wiki/Procellariidae

"According to the famous DNA hybridization study into avian phylogenetic relationships by Sibley and Ahlquist, the split of the Procellariiformes into the four families occurred around 30 million years ago; a fossil bone often attributed to the order, described as the genus Tytthostonyx, has been found in rocks dating around the Cretaceous-Tertiary boundary (70-60 mya), but the remains are too incomplete for placement within the Procellariiformes to be certain.[2] The molecular evidence suggests that the [swimming] storm-petrels were the first to diverge [develop] from the ancestral stock [ichthyornithes], and the albatrosses next, with the procellariids and diving petrels splitting [developing] most recently. Many taxonomists used to retain the diving petrels in this family also, but today their distinctiveness is considered well supported."

Owls

This chart shows that owls have their own separate lineage (within the landbird branch) which supports what I have proposed concerning owls:

The Branching Tree

Many existing bird orders trace back to Enantiornithes niche subgroups.

Quote:

http://en.wikipedia.org/wiki/Enantiornithes so that it is possible that enantiornithines may actually represent successive outgroups on the lineage leading to modern birds. AND http://en.wikipedia.org/wiki/Apsaravis This would mean that, rather than a radiation of primitive birds separate from the radiation that led to modern birds, "enantiornithines" would actually be steps along the way to becoming modern birds.

AND

Quote:

http://en.wikipedia.org/wiki/Enantiornithes Enantiornithine fossils appear to include waders, swimmers, fish-catchers, and hook-beaked raptors. And http://en.wikipedia.org/wiki/Bird Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.

Let us say for the sake of simplicity that there are 3 niche groups within enantiornithes (waders, seabirds and landbirds). Think of these as 3 major branches. From each of these major branches, a number of smaller subbranches branch off. For example, the subbranches off the wader branch lead to the variety of modern wader orders. The subbranches off the seabird branch lead to the variety of modern seabird orders. The subbranches off the landbird branch lead to the variety of modern landbird orders.
It is a branching tree. In this picture there is increasing specializations and expansion within the niche and expansion across the globe exploiting that niche..
For example, the wader groups of Enantiornithes developed into a variety of specialized wader bird orders and expanded across the globe in their niche.

Aves 3D

This is a very cool website showing bird structures in 3D:
Here is an ostrich sternum (not keeled):
http://aves3d.org/specimens/129-Struthio-camelus/specimen_elements/342-Sternum