For the moment I would stick with what you have, you won't see a major difference between the two zooms.
As I found out the hard way, taking photos of small birds is a lot harder than it looks. Size matters and even my Sigma 150-500mm lens is too small, ( until a swan sails past ).
I'm reasonably happy with this Sigma lens, just about affordable, unlike the big lenses from the major manufacturers which would mean doing a part exchange for my car. It's pretty hefty though, fine when you are in a hide but not the thing to stroll around with.
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