Both of the lenses you mentioned are not macro lenses. They are labelled macro because their magnification level is close to 1:1 and they offer decent close-up shots. However, a true macro lens offers a 1:1 magnification level, which means that the object you are photographing appears exactly the same on the sensor as it is in reality.

Now, the lenses you mentioned still offer decent close-up shots (Sigma 70-300 offers approx. 1:2 and the 24-70 approx. 1:3). However, for butterflies and dragonflies (depending on how close you want to get) think about investing into a true macro lens. Here, the focal lengths that have the best working distance are either 105mm or 150mm. Unfortunately, these are rather expensive. Now, if you are using only a macro 1:1 lens (either 105mm or 150mm) this is the best working distance you will ever achieve. Attaching, extension tubes or reversed lenses only makes this distance shorter and the lens longer, which is also the case with the Sigma 30-700mm (maybe very awkward to shoot insects). There aren't any ways to increase the working distance of a macro lens but both 105mm and 150mm offer decent working distance. Also, as a macro photographer you will need to work around this problem and compensate for the short working distance.

If you want to shoot birds, you don't need a macro lens and the 70-300 will be good for it. Most wildlife photographers use telephoto lenses to shoot birds and you can get decent close-up shots with them. Besides, even if you use a macro lens to shoot birds, you probably won't get that close to them and will scare them off.

So to sum up, telephoto converters and extension tubes make the working distance shorter and the lens longer (which is more awkward to handle) and you don't need a macro lens to take pictures of birds. You need a telephoto lens to do that.

Hope this helps.