Conserving urban habitats: Enhanced detection of common swift nests in buildings using a video fusion method

Common swifts ( Apus apus ) nest in cavities and cracks in building facades and roofs. It is a declining species in Europe, partly due to the loss of suitable breeding habitat caused by urban building renovation (e.g. thermal insulation). In several countries, building renovation projects imply quantifying the amount of habitat occupied by swifts that will be destructed, and proposing compensatory measures, such as installing nest boxes in the renovated building. A difficulty is that nesting swifts are uneasy to detect visually. Here, we report a new video method we used to improve swift nest detection. It is based on continuous filming of a building's facade (for 1–2 h) and later video frame fusion to reveal swift flying in and out of cavities in the facade. We successfully used this method on two large building renovation projects, revealing larger than expected breeding populations (78 nests) and resulting in the implementation of ambitious compensation measures (212 nestboxes), commensurate with the habitat actually destroyed. Based on a continuous dataset of swift passages during a full breeding season, we discuss the favourable period in the season and day hours to efficiently detect nesting activity and assess building occupancy. Solution. A video method is proposed to survey bird populations nesting in walls or cliffs. Video frame fusion reveals the fleeting movements of birds to their hidden nests. We provide software to use this tool in conservation projects, in particular ecological inventories prior to building renovation work.