This is a repost from The Northern Ireland Swift Group by Mark Smyth
Many homes and buildings in the UK and Ireland have these air vents in front of or under the eaves. I recently found one lying on the ground. I realised that if the central, 14mm x 14mm, squares are removed the hole is perfect for swifts. With the spikes filed down this leaves a 64 x 28.5mm hole. This will exclude starlings but not tits and sparrows.
'Air terminals' are the supply air outlets and 'return' or 'exhaust air inlets'. For supply, diffusers are most common, but grilles, and for very small HVAC systems such as in residences, 'registers' are also used widely. Return or 'exhaust grilles' are used primarily for appearance reasons, but some also incorporate an air filter and are known as 'filter returns'.
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