I honestly think the birds being in that picture don't mean a thing. Those look to just be stock animals. And they wanted to fill the space in the upper-right with something... Which is the sky. So you gotta put something there. Definitely don't read into that.
However, the animals listed in the files... That means something. It doesn't necessarily mean it's coming soon. But it means those animals are currently, or were at one point, in the works.
EDIT: And regarding the holiday aspect? It would make a ton of sense to release a semi-aquatic DLC pack around the holidays to spike interest in the game again.
I absolutely disagree with that, I think the animals in the files is a clear attempt at Frontier, letting us all know, calm down, relax, the aquatic features are coming. The birds at the top right hand corner are another mild hint, they know fully well that birds are missing from the game, these guys have a very high IQ, they will never place birds on that background if this was not something they will deliver at some point the complete opposite, they would have just avoid anything bird related. Also the Impala on the background savannah, they have plenty of antelopes in game they could have used a number of them, but they picked the Impala. Then there is what looks to me at least like a majestic African Leopard, it could be a lioness, but the way it looks, the anatomy of that predator, the way the tail is up, as an animal artist and modeler it tells me it is a Leopard. Last, there is the Orangutan, doing exactly the type of behavior that is iconic to this species, and the same behavior that is currently missing in the game, the biggest and brightest stain in their animal roster, why even put that there? My apologies, but everything in that background means something. The files are just that, an implied leak, but a very obvious one, because everyone can see them in the scream it is starring at you, not a clue, but a certainty.