There is currently a large Swift population that has been using the church's main chimney as a nighttime roosting place. If you are interested, the chimney swift is a medium-sized, sooty gray bird with very long, slender wings and very short legs. Like all swifts, it is incapable of perching, and can only cling vertically to surfaces.
It builds a bracket nest of twigs and saliva stuck to a vertical surface, which is almost always a human-built structure, typically a chimney.