I have a 1880 house with balloon framing. On the 3rd top floor, the roof line extends beyond the top plates and there's a air gap at the end...I can right into my rain gutters from the inside and have had bird problems in the past.
What do I put here to block light or birds from coming in?
Wire mesh for birds/squirrels (while permitting ventilation) louvers (usually small metal vent louver strips) to block birds and light (to an extent) if blocking light is important to you. If blocking light better than simply using louvers is desired you might need to construct a soffit externally which would typically have the vent louvers in the bottom. Some sort of internal baffle could also be arranged, if more convenient.
Unclear what your insulation and associated ventilation needs are, but typically "cold roof" setups require vents at the eaves and other vents at or near the peak (typically gable end and/or cupola vents on that era building, as built, anyway.)
