Revised January 08, 2018 with a revised cladogram.
cladograms have no trouble nesting closely-related, highly-derived sisters. The trouble shows up when you try to find out how basal clades are related. Sometimes bird cladograms look more like combs than ladders or bushes. Not a problem here in the fully resolved large reptile tree (LRT, 1145 taxa, subset Fig. 1).
the Neornithes is basically halved (given presently tested taxa) at the grackle/crow/blue jay (Quiscalus/Corvus/Cyanocitta) clade. Here the descendants of an unknown prehistoric grackle represent the second half of neornithine evolution. Most, but not all, of those basal taxa have a bigger bill. Most, but not all, have longer legs. The picture stays about this fuzzy until more taxa are added. But this cladogram lumps and splits verifiable relationships better than prior ones.
Figure 1. Subset of the LRT focusing on birds. Here various aspects of birds are shown, including age, teeth, feeding behavior and basic clades.
includes a wide variety of taxa. Here only basal forms are illustrated.
Quiscalus quiscula (Linneaus 1758 ) is the common grackle, what one might consider a fairly typical bird, but it is phylogenetic miniature of its ancestors, the long-legged screamers (Anhima and Chauna). And it’s a better flyer.
Scopus umbretta (Brisson 1760, 56 cm tall) is the extant hamerkop. Basically this is a dull, long-legged grackle with a long narrow bill (Fig. 2). In the LRT the hamerkop nests basal to the shoebill and pelican (Pelecanus), petrels (Macronectes), auks, puffins, New World vultures (Coragyps) and their descendants the dodo and solitaire.
Pteroglossus aracari (Linneaus 1758) is the extant black-necked aracari, a type of toucan. Basically this a grackle with a giant bill. Here toucans are related to Old World hornbills and worldwide barbets.
Threskiornis aethiopicus (Latham 1790, 68 cm long) is the extant sacred ibis. Basically this is a wading grackle. It has a narrow, down-curved beak. It nests basal to the spoonbill (Platalea) and duck (Anas) clade. Unfortunately, that missing link remains missing.
Aramus guarauna (Linneaus 1766) is the extant limpkin. Basically this another long-legged wading grackle. Congeneric specimens of this derived taxon are found in the Miocene.The limpkin is often considerd transitional between rails and cranes, but here nests basal to wide variety of birds: sea gulls, terns, hummingbirds, penguins, woodpeckers, dippers, wrens and pigeons.Smaller bills represent phylogenetic miniaturization. Such variety argues for a deeper origin of neornithes, despite the lack fossils in the Cretaceous.
Figure 2. Quiscalus and its descendants, Scopus, Aramus, Threskiornis and Pteroglossus. Note the gradual retraction of the naris.
When we talk about the gradual accumulation of traits
this is how that manifests itself with available taxa. Before these taxa had descendants that evolved into so many different phenotypes, these five were sisters that took the grackle bauplan and tweaked it one way and another and another world wide.