Both for the welfare of the birds studied and the validity of the results obtained, it is important that biologging attachment methods should be suitably safe and effective. We describe a weak-link wing harness, designed for long-term attachment, which safely detaches from the bird without need for recapture, and a UK field trial on two species of gull with contrasting life-histories, Herring Gull Larus argentatus and Lesser Black-backed Gull L. fuscus. We fitted 15 GPS devices to Herring Gulls in 2014 using three different weak-link materials: cotton thread, cotton piping cord and nitrile rubber. Productivity and return rates were compared against those fitted with permanent harnesses and a control group. A further 36 weak-link harnesses were fitted to Lesser Black-backed Gulls in 2017 and 2018 using GPS-GSM devices that provided more accurate attachment durations. The weak-link design was suitable for Herring and Lesser Black-backed Gulls and harnesses detached completely. Devices fitted with cotton piping cord or nitrile weak-link harnesses remained attached to Herring Gulls for up to four years. Cotton thread was less durable, with harnesses detaching in 1–2 years. We did not detect any significant effects on breeding success or return rates from the harnesses, although small effects sizes cannot be discounted. Devices fitted to Lesser Black-backed Gulls with 6-ply cotton, 18-ply cotton or piping cord weak- link harnesses had respective mean (± standard error) minimum attachment durations of 102 ± 13.6 days (N = 15), 358 ± 42 days (N = 18) and 596 ± 225 days (N = 2). The use of weak-link harnesses can provide a flexible and favourable alternative to permanent designs mitigating some of the associated risks.

The use of biologging and telemetry devices has become widespread in avian ecological research in recent decades (Geen et al. 2019; Ropert-Coudert et al. 2009). While the insights gained from bird-borne devices are well recognised, these must be offset against the potential to cause detrimental effects to individuals. The ethical considerations for those undertaking biologging methods and the importance of not unduly impacting a bird’s welfare have been widely discussed (Wilson & McMahon 2006; McMahon et al. 2011). Any impact on a bird’s welfare may also cause biases in the data collected, undermining study aims. The attachment of devices to birds may have direct impacts on an individual’s health or its behaviour, potentially resulting in impacts to fitness, i.e. reproductive success and survival. Recent reviews have evaluated these potential effects and their impacts; however, there is still a need for improvement in reporting (Barron et al. 2010; Bodey et al. 2018; Geen et al. 2019).

A number of factors may determine whether deployments of devices on birds may affect them, meaning that it may be difficult to predict the likelihood of impacts (McMahon et al. 2011). Thresholds of device mass relative to body mass are most commonly used to assess risk, with devices over 5% of an individual’s body mass most likely to have negative effects (Geen et al. 2019). Relative device masses have not necessarily decreased over time, however, instead newer lighter technology has enabled the deployment of additional sensors within devices and for smaller individuals and species to be studied (Portugal & White 2018). Additionally, the design and influence on drag (Vandenabeele et al. 2012) as well as the attachment methods (Geen et al. 2019) can influence the likelihood of detrimental effects, even if the mass of the devices is under 5% relative to the individual (Bodey et al. 2018).

A shortcoming of temporary attachment methods, such as glue-mounting, is that data are collected only over a brief period and may not represent other times in an individual’s annual cycle. With improvements in device longevity, it is now commonplace to track individuals, of larger species, continuously over multiple years, which typically requires the use of harnesses to ensure the longterm retention of devices (Klaassen et al. 2014). Harnesses have been used widely for many years, particularly on raptors (Kenward 1985), with design varying according to species and the intended position of the device on the bird. Harnesses are generally considered higher risk than temporary methods of attachment (Bodey et al. 2018; Geen et al. 2019) since there may be the risk of injury or direct mortality, for example from entanglement (Foster et al. 1992), if they are poorly designed or fitted, or fall off or become damaged after wear. As with other attachment methods, impacts on breeding productivity have also been reported (Mallory & Gilbert 2008; Phillips et al. 2003) which may be sustained if the device is not shed. The suitability of harnesses and their design can also vary between taxa (Withey et al. 2001). For example, one study found no short-term impacts from the use of wing harnesses on Lesser Black-backed Gulls Larus fuscus (Shamoun-Baranes et al. 2011) and showed this was a preferable design with greater attachment longevity and less feather shading of the device’s solar cells compared to a leg loop harness (Thaxter et al. 2014). However, when the same attachment was used on Great Skua Stercorarius skua overwinter survival was reduced (Thaxter et al. 2015). This is in contrast to Mallory & Gilbert (2008) who reported successful use of leg loop harnesses on South Polar Skua Stercorarius maccormicki, a structurally similar species.

The use of permanent harnesses poses an ethical problem, particularly if recapture is not likely, as individuals may bear a device indefinitely, usually beyond the functional lifespan of the device itself. If during long-term deployments, permanent harnesses wear to the point of failure, this may result in an incomplete detachment, potentially causing injury or mortality through tangling. Using harnesses which incorporate a weak-link, designed to enable the entire harness and device to be safely shed at some future point, provides a favourable alternative to permanent harnesses. Weak-links are not a novel idea and have been incorporated into harness design for decades (Karl & Clout 1987) and are widely used, mostly on raptors (Bedrosian et al. 2015; McIntyre et al. 2009), but also on other taxa (Bedrosian & Craighead 2007; Deguchi et al. 2014; Higuchi et al. 2004; Parejo et al. 2015). However, there are uncertainties about the reliability of weak-link harnesses (Kenward 1985) and evaluation of their use remains scarce in the literature.

Here we describe a weak-link wing harness, designed for long-term attachment, but also to safely detach from the bird without need for recapture. We also evaluate field trials of the design deployed on Herring Gull L. argentatus in 2014 and Lesser Black-backed Gull in 2017 and 2018, species with contrasting lifehistory traits, the former being largely resident and the latter largely migratory. We evaluate both (i) the harness performance, through harness retention rates, minimum attachment duration and assessment of the harnesses on recaptured birds, and (ii) the potential impacts on birds, through inspection of recaptured individuals and comparison of the breeding productivity and return rates between tagged and control groups of Herring Gulls. We also provide recommendations for future use of the design.

The study on Herring Gulls at South Walney and this methodological review were funded through the Department for Business, Energy and Industrial Strategy (BEIS) (formerly the Department of Energy and Climate Change) Offshore Energy Strategic Environmental Assessment programme and our particular thanks go to John Hartley (Hartley Anderson Ltd) for his support of the work and management of the contract. The studies on Lesser Black-backed Gulls at South Walney and Barrow-in-Furness were funded by Ørsted (formerly DONG Energy) and those at the Ribble and Alt Estuaries and Bowland Fells by Natural England and our thanks to all staff involved for their support of these projects.

Thanks to Gary Brodin from Pathtrack and to Phil Atkinson from Movetech Telemetry for additional assistance and technical support with their devices. We are very grateful to Cumbria Wildlife Trust staff at South Walney for assistance with setting up the tracking systems and monitoring birds and their nests and Andy Bates (Leck Construction) for helping facilitate site access in Barrow-in-Furness. Access at the Bowland Fells site was kindly arranged with the Abbeystead Estate. Phil Atkinson, Rachel Taylor, Katharine Bowgen, Lee Barber, Greg Conway and Liam Langley also provided field assistance. We would also like to thank the Special Methods Technical Panel, in particular the convenor, Jez Blackburn for their advice. Mike Toms provided assistance in formatting the online Supplementary Materials.

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Clewley et al. appendix - BTO Gull Harness Protocol [PDF]