Birds on the move

The Yellow Box Eucalyptus leucoxylon ssp. megalocarpa in my garden is covered in dusky pink blossoms, and will be flowering for a while yet as there are lots of buds. The branches touch the ground, and I think that the weight of gum nuts from previous years' flowering could cause branches to break but so far so good. It's very distracting because I can see it from my kitchen and it's outside my study window.
The tree is visited by birds all year round and Red Wattlebirds nest in it each year, but it is in autumn that it becomes a gourmet food palace. It's like a cocktail party or pub crawl-so noisy, as groups of lorikeets and honeyeaters arrive, shout loudly, hustle for position, flit from one tasty area to another, have short noisy conversations with this one or that and then fly off to another party.
Musk Lorikeets are much more numerous this year and are the dominant visitor to my tree. Red Wattlebirds and New Holland Honeyeaters are also prolific, as well as bees and insects. Small flocks arrive, announce their presence loudly, stay for a while, then they're gone.
Why do some birds move quickly from tree to tree and within a tree? It seems to be a wasteful use of energy. Why don't they stay in one tree or even in one clump of blossom until the food there is finished? But no, as I watch they fidget continually. Is it a survival technique? Is it instinctive or learned behaviour to prevent capture by a hawk or a sparrowhawk?