"That the birds of worry and care fly over your head, this you cannot change" suggests that life will always have its share of worries and concerns, and we cannot completely avoid encountering these challenges.
"But that they build nests in your habits, this you can prevent" emphasizes that we have the power to control how we respond to worry and care. We can prevent them from taking root and becoming habitual patterns in our thoughts and actions.
In essence, this quote encourages us to acknowledge that worry and care are part of the human experience. We cannot eliminate them entirely, but we can choose how we deal with them. By being aware of our habits and thought patterns, we can consciously work to prevent negative emotions from becoming deeply entrenched in our lives. It reminds us to practice mindfulness, resilience, and proactive approaches to manage stress and maintain a healthier mental and emotional state. By fostering positive habits and coping mechanisms, we can keep worry and care from negatively impacting our overall well-being and create a more balanced and harmonious life.