Zoom Lens for backpack
Your own suggestions and your question about the ability to shoot birds tells me, that you want a telezoom.
As weight and size is important, and as I think it can be useful for you with a zoom covering as much - in the short and long end - as possible, I think you should also consider either the Canon 55-250 IS or the Tamron 18-270 VC macro.
The canon is small, light and cheap, but (a good copy) has very good image quality.
The Tamron is more expensive and heavier, but it can may be cover most of the f.l.´s you need, so you almost never have to change lenses. The i.q. from the Tamron is not as good as from the Canon (even if you have a good copy of the Tamron.)
I would never recommend you to bring a big, heavy, conspicuous and very expensive L lens when you are bag-packing.