For a while I was occasionally feeding two crows with a juvenile, currawongs and magpies in our yard. About three months later I found a collection of Leopard Tree leafy twigs neatly piled up on the back deck. A few days later another pile appeared. These I cleaned up. A few days later more appeared but this time up on a shade sail above the deck. Again I cleaned them up. Three weeks later there is another pile on the shade sail. I have not witnessed how they get there but have seen the magpies and crows clipping off the same size twigs off a Titree and just dropping them. Any ideas about this behavior....if it was the birds?
At the moment I am finding leafy twigs under my Eucalyptus viminalis trees, the birds break them off to get at the borer grubs inside the branches. In your case there might be a food source involved but the birds are carrying the twigs to your deck to eat insects on the branches - just a guess.
Maybe but why not eat them on the trees? The piles are stacked too neatly to just act as a take-away meal plus we have been here for 30 years and never had this happen before. The plot thickens.....
They are giving you a gift so you give them one in return.