Abstract
Renewable energy, and in particular the development of offshore wind farms (OWF), is one of the key strategies of the Dutch government to fulfill the requirements of the various agreements to combat global climate change. Seabirds can however be negatively impacted by the development of Offshore Wind Farms (OWF) through the risk of direct mortality due to collisions and the risk of indirect mortality due to either displacement leading to habitat loss, or barrier effects leading to avoidance and increased energy expenditure. The Sandwich Tern Thalasseus sandvicensis is one of the species that was identified as particularly vulnerable to the direct effects of OWFs. However, assessing the potential negative effects of OWF development on Sandwich Terns is hampered by a variety of knowledge gaps mainly related to disturbance effects of operational OWFs, leading effectively to habitat loss, but also to some specific parameters that are part of Collision Rate Models (CRM) that are used in assessment procedures. Furthermore, some demographic and life-history parameters of Sandwich Terns are unknown too, leading to a reduced ability to calculate population-level effects.
In this study, the at-sea distribution, and the macro-avoidance of OWFs by Sandwich Terns were thus of primary interest, as these will together determine how often birds encounter an OWF (also related to collision rates) and how much habitat could be lost after construction of OWFs. GPS-tracking allows to study behavioural responses to the presence of OWFs in detail, including macro-scale avoidance as well as many aspects of the actual flight behaviour, such as speed, flight altitude and behaviour. Therefore, a main component of the studies reported in this report is based on GPS-tracking of individual Sandwich Terns. Avoidance of OWFs may also mean that movements of individual birds are compromised, which could lead to higher energy expenditure or less profitable prey availability. Habitat loss may thereby affect diet, which in turn may affect reproductive success through so-called carry-over effects. Therefore, baseline data on Sandwich Tern diets has been collected in this study. Furthermore, annual survival of Sandwich Terns is a primary parameter in population modelling, which was studied with colourrings. It is also important to estimate how many individual Sandwich Terns can interact with an OWF over longer time spans than just a single breeding season. Therefor, immi- and emigration rates of different populations were studied.