Just balance the weight, I was thinking along the lines of carrying a 500ml bottle of water along, Im sure 500grams would be fine. Alternatively you could get her to 'carry' something in her mouth - its about both making the walk physically harder, and stimulating her mentally to do some work.
Ah collie cross, very intelligent dogs, love to do a bit of herding - its a difficult one to design an activity yourself for. That may explain the bird barking, they can decide that they want to herd the birds and bark to signal to them to get a move along!!
Theyre good with the nose too (as are terriers) so you could try a chicken smoothie activity, get some chicken grease, mix with water, into a squeezy bottle and make trails for her to follow when your out walking - so youd spray the chicken grease mix in a long trail with something for her to 'find' at the end of it. Obviously lots of affection at the end of the find. Youd need to set the trail up out of her sight - its a good mental and physical activity.
Or you might think about training her to do something in particular when she sees birds, as opposed to her just barking, the training could consist of her having to go get something for you when sees a bird, and this could be escalated to her having to move something along the garden (like a large light ball maybe - something that would be easy for her to move, but enough of a movement to frighten off birds?).