I don't know what kind of bird last inhabited this old structure but I like to think it was a sparrow.
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Sparrows are said by many cultures and beliefs to be Psychopomps. Psychopomp means the "guide of souls"; they are creatures, spirits, angels or what have you, whose responsibility is to escort newly deceased souls to the next life. According to the ancient Egyptians sparrows caught the soul of a person recently deceased and carried it to the afterlife. In many European cultures, a sparrow flying into the home is said to be a sign of impending death.
In this case, the sparrow fled.