Offshore wind energy - sectoral marine plan: seabird tagging feasibility

Tag types

Species considerations

There are a number of restrictions in relation to the types of devices that can be deployed on different species. The most widely known of these is the “3% rule” (Bodey et al., 2018; Geen et al., 2019), which states that tags must be no more than 3% of a bird’s body weight in order to minimise the risk of deleterious tag effects (Table 2). However, there are a number of other restrictions linked to the behaviour of the species concerned which influence how, and where a tag may be deployed (Vandenabeele et al., 2014, 2012, 2015). Typically, there may be requirements for leg or tail-mounted devices to be lighter than back-mounted devices (Table 2).

¹Birds of the Western Palearctic Vol. 1 Ostrich to Ducks ²BTO Birdfacts ³This value exceeds the 3% threshold of the upper weight range for kittiwake, and is likely to relate to a previous 5% threshold. Such a device is unlikely to be licensed at present.

Other species-specific considerations include:

Gannet: The tag must not impede their ability to dive, so tail mounting is most common. It must be waterproof to 30m and capable of withstanding impact. Tail mounts are temporary, so it should be assumed that data collection will be short-term. Keeping feather waterproofing intact is essential.

Herring Gull: Feathers are weak, so devices taped to feathers do not last long. Consequently, harnesses are preferable. As recapture is very difficult, archival tags are not suitable.

Great Black-backed Gull: Feathers are weak so devices taped to feathers do not last long. Great black-backed gulls have been shown to not tolerate harnesses so this method is unlikely to be suitable for tag deployment. As recapture is very difficult, archival tags are not suitable. Recent trials of tags on great black-backed gulls, both in the UK and Canada suggest that there is a high likelihood of deleterious tag effects (Maynard et al., 2022, 2021).

Kittiwake: Kittiwake may have a high likelihood of device effects (e.g. Clewley et al., 2022), consequently, devices should be kept as light as possible to increase chances of representative data. There is clear evidence of potential device effects from harness-mounted devices (Clewley et al., 2022) and concern about potential device effects from glue-mounted devices (UKCEH & RSPB unpublished data). Tape-mounted devices attached to birds tails, or leg-mounted devices, are believed to be least problematic at present.

Guillemot: Keeping feather waterproofing intact and maintaining neutral buoyancy is essential. As Harnesses and glue-mounting do not allow for this, tesa tape mounts to the back are recommended. Device must be light weight, hydrodynamic and waterproof to 150m+.

Razorbill: Keeping feather waterproofing intact and maintaining neutral buoyancy is essential. As Harnesses and glue-mounting do not allow for this, tesa tape mounts to the back are recommended. Device must be light weight, hydrodynamic and waterproof to 150m+.

Puffin: Does not readily accept tags, and many back mounted devices have caused issues (e.g. Harris et al., 2012). Keeping devices to the minimum size possible is recommended. Small leg-mounted devices are recommended. Device must be light weight, hydrodynamic and waterproof to 70m+.

Data considerations

The study aims will play a key role in determining which type of tag is chosen. At a high level, there may be a choice between using a geolocator, or a GPS tag. Geolocators estimate a birds location based on light intensity, with daylength used to estimate latitude and the timing of dawn, dusk, and midday used to estimate longitude. They tend to be cheaper and lighter than GPS tags and can be mounted on a leg ring so are less likely to cause tag effects than heavier tags mounted on a bird’s body. However, the spatial and temporal resolution of the data are far lower than is possible from GPS tags, with errors typically in the region of ~40 km and fixes limited to 2 per day (Rakhimberdiev et al., 2016). Consequently, geolocators tend to be favoured for collecting data on the distribution of birds outside the breeding season, particularly for smaller species (e.g. Buckingham et al., 2021), whilst GPS tags, which may be restricted to shorter-term attachment methods, are used for assessing distribution during the breeding season (Thaxter et al., 2015). However, geolocators do not provide information on a bird’s latitude during the equinox due to daylength being constant at all latitudes. They are most useful for providing information on long distance migrations and an approximate location in winter. They are not sufficiently precise to determine whether a bird is within an offshore wind farm or not. Platform Transmitting Terminal (PTT) are also capable of estimating position using the Doppler effect and polar-orbiting satellites. However, the triangulated positions are only accurate to within >500 m and can only be estimated a few times a day. Consequently, as they lack the spatial and temporal resolution of GPS tags, and the advantages in terms of size and cost of geolocators, they are not considered further here.

Both GPS tags and geolocators can record additional data, and can be deployed alongside other tag types (e.g. altimeters and time-depth recorders) to provide further insights into bird behaviour (Table 3). Of particular value in the context of assessing impacts of offshore wind farms on seabirds is the potential for GPS tags to record information on flight heights (Ross-Smith et al., 2016) and speeds (Masden et al., 2021). It is also useful to note that manufacturers are looking to integrate different tag types, with notable examples including geolocators with integrated time-depth recorders, and GPS tags with integrated altimeters, time-depth recorders and/or accelerometers.

The volumes of data collected by GPS tags can have important implications for battery life. Many tags now come equipped with solar panels in order to help prolong battery life. To maximise the value of this, it is important to consider attachment methodologies. If feathers or tape obscure the panels, this will impair the ability of the tags to recharge and is likely to have implications for battery life. Consequently, solar panels are unlikely to be an advantage for species such as gannets or auks, where tags are deployed using tape.

An additional means to prolong battery life is the potential to use geofences. These can be used to vary the rates at which data are collected based on the location of the bird. For example, these can be set to record at a lower rate, preserving battery life, when the bird is at its nest. Conversely, a higher sampling rate could be set for periods when a bird is in a wind farm and higher resolution data to capture the movements of the birds in response to turbines may be valuable.

Additional tag considerations

Additional considerations in relation to tag types relate to data retrieval and cost. In the case of geolocators and some time depth recorder tags, the bird must be recaptured in order to retrieve the tag and the data it holds. However, for GPS tags, the remote download of data, either to a base station or via the mobile phone (GSM) network is possible. The use of remote download tags significantly increases the chances of data retrieval, which may otherwise be subject to factors including bird mortality or movements away from the breeding colony (e.g. due to breeding failure), restrictive weather conditions, the potential impacts of HPAI, or birds becoming more wary of fieldworkers, and therefore harder to catch. For these reasons, archival GPS tags are often retrieved well before the battery is exhausted, limiting the quantity of data which may be collected. Furthermore, as data can be retrieved without having to re-enter the breeding colony, the use of remote download tags can also reduce the overall disturbance to that colony.

Remote download tags do however bring additional challenges. Firstly, the power required to transmit the data when GPS is combined with other sensors such as accelerometers means that a larger battery is required than if an archival tag were used. This potentially limits the spatial and temporal resolution of the data collected and/or the species on which these tags can be deployed. The use of solar panels can alleviate these issues but may introduce secondary issues including limitations on the attachment methodology and the potential for increased device effects. Where data are transmitted over the GSM network, this will incur an additional fee due to the associated network charges. Whilst tags which transmit over the 2G and 3G networks may be cheaper, these are being phased out meaning that, particularly in more remote locations, coverage may be severely restricted. Consequently, despite the additional cost, tags which transmit over the 4G network are preferable. Where tags transmit to a base station, it is necessary to find a secure location for this base station, where it will not be at risk of theft, vandalism, or damage from environmental conditions (e.g. strong wind). Mobile base stations are available from many manufacturers, though these will incur additional costs in relation to staff time. Devices are also available which are capable of transmitting data through the “Internet of Things”. However, as this requires them to be close to other devices connected to this network, at present, this technology cannot be considered reliable in remote areas.

Further details of available tag types are given in Appendix 1.