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Tin foil strips may help, as well as hanging old CDs. The flashes of light off the shiny parts will startle the birds but eventually they do become accustomed to it. Netting is the best protection as it prevents the birds from getting to the tree, so perhaps you could ask a neighbor to help. Here is an article on some other things to try: https://www.gardeningknowhow.com/edible/fruits/fegen/protect-fruit-tree-birds.htm

You can try treating the lawn with Diatomaceous Earth or neem oil. Both are safe for animals but kills insects. These articles have more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/diatomaceous-earth-insect-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/mole-cricket-control.htm

Please check the web for some of the newer products that help kill fire ants.
Hopefully this article will be of some help:

https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-planting.htm

Your feeder has to have an air leak somewhere or is hanging crooked , Take a straw in a fountain drink,cover the end and take it out. It should stay full till you remove your finger or turn sideways where air can displace the liquid. If it has a cork stopper it might be getting air inside.

Nearly any type of tree will attract birds, if for nothing else, it provides them with shelter. They particularly enjoy fruit trees or anything that produces berries (which are a nice treat in winter). These articles will help get you started in selecting a suitable container tree: https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm
https://www.gardeningknowhow.com/edible/fruits/fegen/dwarf-fruit-trees-a-planting-guide-for-fruit-trees-in-containers.htm

In addition, you could also plant sunflowers in the area or add a water feature, like a birdbath.

It depends on what kind you used. Some fertilizers and pesticides could be harmful, while others would not be. Do you remember what it is you used? I would be better able to tell you the effects if you can let me know that.