You tried different ways to keep the tiny bird warm and secure. Both were started with the best in heart but the worst in mind and basic understandings of gravity. A rock was too heavy for the bird, and you realized that quite quickly. The heaviness of your body (which was much heavier than the rock) though? You thought that would be fine, which is incredibly strange. You are a kind bear, but you really need to think things through more thoroughly.
Hopefully, however, you have learned a very important lesson in life: it is not good to put very heavy things on very tiny things to keep the tiny things moderately warm.
Life is about learning.
Would you like to try keeping the baby bird alive by not placing heavy thing on it?