Example of tech. drawing from the book "Bouwprojecten en Gierzwaluwen in Amersfoort" by Marjo van der Lelie. The book contains a lot of examples of projects in the city of Amersfoort in which nesthelp for Swifts was incorporated. The book can be ordered at the publisher (+31 033 4613630 or c.f.vanderlelieAThcc.net (replace AT by @).