VERKADE H, JACOBS J, MARIJNIS AS & VAN DIJK I (2015) Monitoring of Swifts Apus apus in Noordwijk by counting nest entrances. LIMOSA 88 (4): 164-172.

Swifts were monitored by counting nest entrances in the village of Noordwijk, Zuid-Holland. Counts were conducted in the late afternoon and early evening, when adults return to their nest to spend the night. Noordwijk consists of two parts. The original village Noordwijk-Binnen (13 000 inhabitants, Fig. 1), was monitored in 1993, 1998, 2003, 2008 and 2013. Noordwijk aan Zee (9 000 inhabitants), a seaside village, was surveyed in 2010. In the year preceding a count, reconnaissance rounds were made to locate colonies and nest entrances.
Over time, the number of nest entrances used by Swifts in Noordwijk-Binnen was more or less stable (148-172 apparently occupied nest sites, Tab. 1). Most nests were situated under roof tiles, behind the roof gutter and in wall cavities; the use of roof tiles has decreased over time. During the study period the distribution of Swifts became more scattered (Fig. 2), with fewer nests per occupied building and fewer nests in older buildings (built before 1940, Fig 3). This change in distribution can be attributed to the renovation of pre-World War II flower bulb sheds with a traditional roof tile type and structure that provided many potential nest sites. Nest entrances were located at a height of 3-14 m above the ground, and were mostly orientated towards the east (1993-1998) and north (2003-2013) (Tab. 4). About 50% of the nest entrances occupied during a certain survey was still occupied during the subsequent survey (i.e. after five years, Tab. 3), resulting in an annual apparent site-fidelity of 87%. In Noordwijk aan Zee 107 occupied nest entrances were found in 2010. Interestingly, the ratio between the number of breeding Swifts and the number of inhabitants of villages with detailed counts seems to be rather constant around roughly 1 Swift per 100 inhabitants (Tab. 5). If we use this ratio to extrapolate to the whole of the Netherlands, Swifts might be four times as common as hitherto believed. However, this extrapolation is based on surveys in relatively small villages, hence there is a strong need for reliable survey data from larger cities.

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