“It is a sign of rain,” wrote the author of the fourth-century-bc Greek treatise On Weather Signs, “if a hawk perches on a tree, flies right into it, and proceeds to search for lice. Also, when in summer a number of birds living on an island pack together, if a moderate number collect, it is a good sign for goats and flocks, while if the number is exceedingly large, it portends a severe drought.”