why do red wattlebirds relentlessly chase weebills

Here in wilmington SA, the red wattlebirds seem to have it in for the weebills.  In thinking about this I have some ideas:
– food competition in terms of insects, although the hugely different size of the two birds makes me wonder if they are eating different insects.  There is a report in HANZAAB of weebill feeding its young a lerp and I imagine red wattlebirds also eat those
– Weebills tend to be in top or upper section of the tree where the red wattlebirds tend to be.  So do yellow thornbills which we also have but don't seem to get the same attacks.  The chestnut rumped and yellow rumped are more out of the way.
Anyone have any ideas?
Approx two weeks ago, pallid cuckoo turned up and my wife  heard Horsefield's bronze cuckoo yesterday.  We have yellow rumped thornbills nesting in the yard.  Three magpies nesting within a few hundred metres of each other.   A few weeks ago, I saw a Raven take a stick to a nest but it appears a magpie has taken that over now.
Gary

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