Bird Nests

You can't prevent a bird from flying overhead, but you don't have to let it build a nest in your hair.

That is an adaptation of a Chinese proverb.  No, it is not scripture. It is what it is; so we won't look for any deep meaning but will take it at face value. At face value, it is a common sense statement.

However, birds are used as symbols in scripture. Their symbolic meaning varies with the species: the dove is identified with the Holy Spirit, a hen with safety and the gathering of her chicks, the ostrich is unmindful, the peacock displayed Solomon's wealth,  however, the devouring birds, those which snatch up seed or feast on carrion, are identified with demons and the devil.
Matthew identifies the birds of the air as "the wicked one" (Matthew 13:19).
Mark connects them with "Satan" (Mark 4:15)
Luke links them to "the devil" (Luke 8:5, 12).

In Genesis 15:11, birds of prey attempted to thwart Abraham's preparation to cut covenant with God. It is prophesied that birds will continue to live down to this foul symbolism in the future when Babylon falls and "becomes a dwelling for demons and a haunt for every impure spirit, a haunt for every unclean bird..." Revelation 18:2

As regards the proverb, "you can't prevent a bird from flying overhead," a person cannot prevent a demonic spirit from suggesting a foul, ungodly thought or putting an evil notion in his head. But he can brush it away immediately and not let it build the metaphorical nest. He can prevent his mind from becoming the home for doubts that question and undermine what the Lord has already said. Blocking the building of the nest is certainly possible.

John 14:1 says, "Do not let your hearts be troubled."  This is akin to saying don't let wicked thoughts find a home in your heart; do not let them build a nest there. Troubling thoughts come from the devil. Always. God's thoughts are peace and righteousness. Devilish thoughts are upsetting and faith-destroying. 

Not all thoughts are benign. Not all thoughts are harmless. Thoughts are not powerless. Thoughts that do not come from God are dangerous. Thoughts that come from devils are destructive and deadly.

The Lesson
A person cannot dwell in a nest of devil-inspired thoughts and still walk out God's plan for her life in faith. She won't have enough courage left to walk the path God has laid. Instead, she will run off in the middle of the night in the opposite direction, the wrong direction.