ST. LOUIS — For months, a murder of crows has been plaguing the federal courthouse in downtown St. Louis and a small public park across the street, leaving sidewalks, grass and benches covered in droppings. They have so far thwarted the efforts of federal officials to get them to leave, but they have also been dropping dead, prompting speculation about the cause. On Monday, a Post-Dispatch reporter and photographer separately counted three dozen dead crows, mainly in the park across the street from the courthouse. Others were scattered on the courthouse grounds or in a small park nearby. There…