Above: Classic Caudipteryx model, from National Geographic.
Ever since I started drawing prehistoric animals for serious, I've been on a crusade to see that it's done right. Too many people draw feathered dinosaurs with zero background knowledge of how wings and feathers are put together or appear in life. One of my biggest pet peeves has been drawings of Caudipteryx which give it secondary feathers. That is, remiges that stem from the ulna. You see, Caudipteryx was one weird dinobird. It had fairly puny wings compared to its body size, with primary feathers only a little longer than the length of the hand. And, apparently, no secondaries. This gives the effect of small, narrow wings good only for flicking out in possibly short displays of small color flashes. All this is based on the holotype specimen of the type species, C. zoui. See my recent reconstruction above right.
However, I recently (finally) got my hands on the descriptions of the referred C. zoui specimen, C. sp., and C. dongi. The referred specimen of C. zoui (BMP 0001) shows the exact same arrangement of feathers as the type. C. dongi, on the other hand, aside from some subtle differences in postcranial proportions that may or may not allow it to be placed in a separate species, has something else: secondary feathers. Check it out:
Above: Portion of plate 1 from Zhou & Wang, 2000.
The photo is pretty poor quality, but you can clearly make out feathers coming from an area inboard of the wrist. The accompanying line drawing makes it more clear:
Above: Portion of figure 1 from Zhou & Wang, 2000.
Notice that in the photo, you can make out possible remains of a tightly banded color pattern also evident in the tail of the C. zoui holotype, but only on the secondaries.
So could C. dongi merely be a growth stage of C. zoui with extra wing feathers? Thanks to two specimens of Similicaudipteryx yixianensis (which are nearly identical to C. sp. in size, proportions, skull shape and even feathering), we know that at least some caudipterids possessed only primaries when they were juveniles and grew secondaries as they matured (the C. sp. specimen also has secondaries in the same proportional size). However, C. zoui is larger than the C. dongi specimen (and the secondary-possessing Similicaudipteryx/C. sp. specimens), so even if it is immature, it must be an immature specimen of an even larger species.
Keep in mind this is all based on published figures and rather poor photographs, so at the risk of sounding like Dave Peters, it would be great for someone with access to the physical specimens to confirm or deny this stuff. Sadly, very little about the actual feathers of the various specimens has been described in the lit, except for the holotype specimen, and even there, seemingly important things like the color bands are only mentioned in a figure caption. At any rate, feather coloration, length, and extent should be kept in mind when trying to determine which of these things, if any, represent distinct species.