BULT H (2002) Nightjars Caprimulgus europaeus in the SW Netherlands. LIMOSA 75 (3): 91-102.

During 1979-97 the numbers and distribution of a little known Nightjar population on the southern part of the "Brabantse Wal" in the SW Netherlands were studied. Aspects of the mapping method, such as speed of the investigation and the use of sound play back, appeared to have a great impact on the final result and could introduce 50% underestimation. To obtain reproducible and accurate results it was necessary to divide the study area in plots with a restricted size of suitable habitat (<35 ha), that could be covered at low speed (15-30 ha/h) in one evening. Selective play back of song, that was stopped immediately when a Nightjar reacted, was often required to elicit territorial activity of isolated pairs (no neighbours within 300 m). Calling males were considered to be territorial until 45 min after sunset. The average minimum size of nine territories, determined by plotting song perches and patrol flights, was 3.6 ha; their greatest diameter was 229 m. Based on that value it was decided to allocate registrations on different evenings that were more than 300 m apart to separate territories. From 1979 until 1990 the population was estimated at 45-50 pairs and appeared to be stable. Between 1991-94 and 1996-97 the population increased from 45 mapped territories (estimate 55 pairs) to 59-61 territories (estimate 70 pairs). The best areas contained 10-14 pairs per km2. Most territories were situated in young plantations and the population increase was due to colonisation of recently planted clear-felled woodland. Since these plantations are abandoned after about 21 years, it is expected that the population will decline in the future.

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