Temporal patterns in offshore bird abundance during the breeding season at the Dutch North Sea coast

The expanding development of offshore wind farms brings a growing concern about the human impact on seabirds. To assess this impact a better understanding of offshore bird abundance is needed. The aim of this study was to investigate offshore bird abundance in the breeding season and model the effect of temporally predictable environmental variables. We used a bird radar, situated at the edge of a wind farm (52.427827° N, 4.185345° E), to record hourly aerial bird abundance at the North Sea near the Dutch coast between May 1st and July 15th in 2019 and 2020, of which 1879 h (51.5%) were analysed. The effect of sun azimuth, week in the breeding season, and astronomic tide was evaluated using generalized additive modelling. Sun azimuth and week in the breeding season had a modest and statistically significant (p < 0.001) effect on bird abundance, while astronomic tide did not. Hourly predicted abundance peaked after sunrise and before sunset, and abundance increased throughout the breeding season until the end of June, after which it decreased slightly. Though these effects were significant, a large portion of variance in hourly abundance remained unexplained. The high variability in bird abundance at scales ranging from hours up to weeks emphasizes the need for long-term and continuous data which radar technology can provide.