|click to enlarge|
Another reason why a concave may be a good idea is that it is not uncommon for Swifts to move their eggs out of the nest, after which, they have no chance of hatching. Sometimes Swifts do this deliberately, because cold, wet weather gives them problems finding food, but other times they do it accidentally by knocking their eggs out of their nests with their long wings when they turn. If the Swifts have a deep concave, this should reduce accidental loss.
Swifts in rehabilitation will climb up on any platform provided, showing that they prefer a platform to a flat surface (per Deborah Lauterpacht).
When a pair of Swifts occupies a nest box with a concave, they invariably use it. In St Mary's Church, Ely, in 24 boxes in 2 cabinets, half have concaves. In 2010, 7 boxes with concaves and 3 without concaves contained nests. The statistical probability of this result occurring by chance is 10.7%. This is indicative that Swifts prefer a nest box with a concave, but to be sure, we need more data.
Concaves can be made of MDF, cork, wood, woodcrete, plaster of Paris, Polyfilla or papier maché [also see how to make them out of Modroc] ; it is a platform 15-35mm high, with a shape scooped out of it 80-90mm in diameter and 10-20mm deep. Or you can use a budgie concave from a pet shop.
Preparing concaves by sticking feathers onto them is a good way of getting children involved. It is one of the activities we give children in our schools projects.