Well as you can see, bg will be a whole world unto itself, and will take the same kind of practice that brought you to the mastery you have in the birds.
It will take a lot of observation of value, for instance you have the same values in the sky as the water, and they are usually very different from each other depending on the light and time of day. Water reflects the sky and land around it etc.
One of my favorite and most useful books on all this is Landscape Painting by Birge Harrison, written in the early 1900's when artists really had great training. This book can be downloaded free, as a PDF. He was an artist, but more important a teacher for 25 years so he knows how to inspire and instruct. It is a delight to read, and really opened my eyes in observation. In the end we will only be able to paint what we can see, but to see it in the first place is the sticky wicket. He really helps one see.
http://www.archive.org/details/landscapepaintin00harruoft It's a great read.
I'm sure you'll also find other artists who you like and see how they work this issue, there are lots of ways to go about it asKen says, maybe for you, it might make sense to start with a vignette, the kind of thing that suggests the immediate background by the bird and fades out to white as you move away from the subject. This style is a tried and true for bird artists. Or try Menaboni's Birds for an example. This would allow you to enter the bg area gently and give you a balance of style closer to what you are used to using.
Later you could branch out into a full background, here are a couple of Menaboni I pulled off the web to show you what I mean. This is just offered as a suggestion to consider.
