Optimising monitoring times for surveys of rūrū (Ninox novaeseelandiae novaeseelandiae)

Automatic recording devices (ARDs) are becoming increasingly popular as a monitoring tool, especially for cryptic or nocturnal bird species. We wanted to determine the optimal time(s) of night and
month(s) of the year for monitoring rūrū (a small NZ owl) using ARDs, to enable development of a robust monitoring method that maximises probability of detection. Fourteen ARDs were placed at 500 m intervals throughout a 400-ha forest block in the Eglinton Valley, Fiordland, New Zealand, and the presence of rūrū recorded monthly from August 2011 to March 2012. Data from the Department of Conservation’s national Tier 1 Monitoring Programme (2011–2016), gathered from multiple locations across public conservation land (PCL), were also analysed to provide broader context. Both at the Eglinton Valley and across New Zealand PCL, rūrū calls were detected in all months surveyed and during all periods of the night. Detection probability (derived from call activity) was generally higher in the North Island and the west coast of the South Island. At the Eglinton Valley the highest call activity occurred in December, with activity in spring generally higher than in summer. On PCL, calling activity was highest in spring and early summer and lowest in late summer. Patterns in how call activity changed throughout the night varied from month to month in the Eglinton Valley. Across New Zealand, PCL (for all months), the call activity peaked approximately 1 hour after sunset then steadily declined throughout the remainder of the night. Our results and those of previous studies indicate general patterns of calling activity but with local variation. We recommend that as a general rule, detection probability can be maximised by carrying out rūrū monitoring surveys in the spring or early summer, during the first few hours after sunset. However, we also recommend an initial study of the site(s) to investigate local variations in call activity before any long-term monitoring is initiated.