If birds perch on a thatched roof, they will most likely start pecking at it, looking for insects, and they may also possibly use the reeds for their nesting material in the spring. Therefore, these spikes will deter them from landing on the most obvious and easy place to perch.
Relevant image keywords:
architectural styles, architecture, architecture and buildings, blue, blue sky, building exterior, building feature, built structure, chicken wire, chimney, chimney flashing, chimney pot, chimney stack, cottage, country life, craft, detached house, deterrent, ecologically friendly, english culture, exterior, flue, guard, home exterior, house, idyllic, layering, lead flashing, man made structure, material, mesh, metal, no people, old, old-fashioned, outdoors, real estate, reed, reed thatch, residential structure, rethatched, roof, roofing, rooftop, rustic, sky, smokestack, straw, tall, thatch, thatched, thatched roof, thatching, traditional, traditional culture, vegetation, vent, white, wire mesh