 The males sing throughout much of the construction and even more so when a female approaches his nest . Following copulation , the male and female continue to build the nest . Nests may be in any type of hole , common locations include inside <unk> trees , buildings , tree stumps and man @-@ made nest @-@ boxes . S. v. zetlandicus typically breeds in crevices and holes in cliffs , a habitat only rarely used by the nominate form . Nests are typically made out of straw , dry grass and twigs with an inner lining made up of feathers , wool and soft leaves . Construction usually takes four or five days and may continue through incubation . 
