Great tits (parus major) have a knack of hiding their nests in plain view, and a gap in a wall seems to be a favoured location. A gap on the top of a cavity-block wall in south Galway was the location chosen by the great tit pictured above. The approach was always the same – fly to a tree on the other side of the road from the wall and have a look around. If all seemed clear, fly across the road to a young ash tree growing next to the wall. Another look around and then straight into the hole in the wall. Both parents keep a constant supply of food to the nest with mouthfuls of caterpillars, craneflies and other creepy-crawlies. On the way out, the birds took mouthfuls of droppings from the nest, to keep it clean.
It was hard to believe that the area in question was heavily flooded last Winter – I saw sandbags around a house around the corner from the nest that seemed to be on quite high ground. This video link shows one of the birds entering and exiting the nest.