After a quick look at your portfolio, it's clear that you aqre intent on following hte learning curve in a very challenging area. One I haven't even thought of trying to work in! You have my admiration for making hte attempt at all!
There's solid technical advice above from Willie and Paul, and Paul is speaking from personal experience, too. I can only add some general thoughts about any specialist area you begin to work in.
First, just getting the gear is a start, but specialist equipment usually needs specialist operation. A Ferrari is very fast, but to exploit the massive performance you need to develop driving skills of a different type (and level) from those needed for a small saloon car. Those skills are not there for the taking - they need to be thought about, developed, and practiced.
So, for your first outing in your new Ferrari, you woud probably choose a warm, dry day, rather than one when the rain was pouring down. Similarly, for natural history pictures, a sunny day when your exposure can be 1/1600 @ f/11 and 800 ISO. And use every aid that you can manage, including a good tripod (and there's a whole further discussion about those, too. If it's light and plasticky, it probably won't do a great job. Big, heavy, and probably expensive wins the day...)
To continue the car analogy: I own an ageing but cheap MG, and it needed new tyres on the rear. I thought, since I bought it for fun, and shorter journeys, that cheap tyres would be OK - makign cornering "more interesting" - in fact, they make driving anywhere on damp roads a distinctly edgy experience. With the right rubber, the car might demonstrate the advantages of mid-engined cars. As it is, it takes nerve and caution to go out in the wet.
Similarly for your Sigma lens: I suggest, first, practice under excellently favourable conditions: and also, plenty of practice: extra frames aren't costing you anything, so shoot plenty of pictures of every scene. Do as the testers do, and practice technique on boring subjects, so that it all works perfectly when there's a bird in front of the lens. (And that's where the car analogy breaks down: there aren't too many places you can practice getting a corner badly wrong, and spinning off into the rhubarb...)