I investigated the role of tide and time of day on feeding success and prey species of the Great Northern Diver Gavia immer at its wintering grounds in Argyll, Scotland. Focal animal sampling was used on solitary Divers to determine their activity during different tidal states and at different times across four main sites. When a bird performed ten successive dives, feeding success was recorded and inferred. Divers spent more time feeding early in the morning in comparison to other periods of the day. Less time was spent feeding at high tide, but this difference was not statistically significant. Divers spent 55% of the daylight period feeding, with most of this feeding time spent underwater. Prey was brought to the surface during 15% of dives and birds drank (thought to indicate ingestion of prey underwater) following 33% of dives. Crabs and flatfish were the main observed prey items, with 61% of the prey brought to the surface estimated to have a mass of less than 5 g, although items up to 80 g were consumed on occasion. The composition of prey brought to the surface varied between sites, with more crab prey items seen on sites with rocky substrates. I investigated differences in the feeding behaviour of adult and first-winter Divers at one site, but observed few differences. The importance of high-quality feeding sites for Great Northern Divers, and the implications of time spent underwater within current census techniques are discussed.

The Great Northern Diver Gavia immer (hereafter 'Diver') is a member of the Gaviidae family that commonly breeds throughout northern North America, Greenland and Iceland. In the eastern Atlantic, its wintering range spans Iceland to the Faroe Islands, Norway and Sweden, Spain and Switzerland, the Republic of Ireland and the United Kingdom (Svensson et al. 2009; BirdLife International 2015). The wintering population of Divers in Scotland (based on the biometric measurement of skins in the National Museum of Scotland) is thought to comprise birds from Iceland (45%), Greenland and Baffin Island (45%) and mainland Canada (10%; Weir et al. 1997).

Furness (2015), recognising that there was moderate uncertainty in the size of the Diver population wintering in the waters around Britain, suggested a figure of 4,000 (adults and immatures), of which 3,000 were around Scotland. Lawson et al. (2015, revised in 2018) used a variety of survey techniques to provide a mean peak Feeding ecology of wintering Great Northern Divers in Argyll, Scotland population estimate of 4,689 wintering Divers in inshore areas around Scotland. The latest land-based survey (Non-Estuarine Waterbird Survey III) provided an estimate of 4,326 for the UK, of which 4,065 were around Scotland (95% confidence interval = 3,388–4,805; Austin et al. 2017; Frost et al. 2019). Estimating wintering populations of Divers is not straightforward; several techniques are used, including aerial surveys, land-based counts and boat surveys, each of which has limitations. Notably, a key UK review of Diver numbers using aerial surveys did not include a correction factor for birds which were underwater during the survey periods (Lawson et al. 2015, revised in 2018). However, a correction factor has been developed for Great Northern Divers wintering off eastern north America (Winiarski et al. 2014).

Scotland, therefore, holds the bulk of the wintering European Diver population (which is estimated at around 6,000 birds; Pennington et al. 2004). It is reported that around 20% of the European wintering population of Divers occur in the waters around Argyll, Scotland (ap Rheinallt et al. 2007), and recently the areas around the Sound of Gigha, and Coll and Tiree have been designated as Special Protection Areas (SPAs) for nonbreeding birds of this species, along with Eider Ducks Somateria mollissima, Slavonian Grebes Podiceps auritus and Red-breasted Mergansers Mergus serrator at the Sound of Gigha (NatureScot 2022). The seas around Argyll are not immune to development pressures such as expansions in aquaculture, changes in fishery practices, and the development of offshore renewable projects (Aitchison 2021; NatureScot 2022). Consequently, a better understanding of the ecology of these species is required to help inform conservation decisions. However, the detailed information on Diver feeding ecology (e.g. time spent feeding and their diet) required to make informed assessment of the risks associated with these threats is currently limited.

Previously, studies of the feeding behaviour and prey of Divers in their wintering habitats around the coastlines of Great Britain have been based mainly on timed dives and some observational studies of their prey. These studies are often based on small sample sizes with limited geographic detail (Collinge 1924; Robinson 1924; King 1976; Kinnear 1978). The duration of Diver dives is influenced by water depth (Kenow et al. 2018) and turbidity (Thompson & Price 2006), as well as choice of available prey and the abundance of prey items (Alvo & Berrill 1999; Gingras & Paszkowski 2006). Studies of the activity cycles of wintering Divers in the western Atlantic found variation in the influence of time of day on feeding activity. For example, at Assateague Island, Virginia, USA, where there is a large tidal range, McIntyre (1978) found an association between Diver feeding and both the time of day and the stage of the tidal cycle. However, at Weekapaug, Rhode Island, USA, where there is a low tidal range, both Daub (1989) and Ford & Gieg (1995) found no association between feeding activity and the tidal cycle or time of day. These studies also found differences in the time that birds spent foraging, with Divers foraging for 55% of daylight hours in Virginia and 30–40% of daylight hours in Rhode Island (McIntyre 1978; Daub 1989; Ford & Geig 1995). In southwest Norway, Byrkjedal (2011), using 30-minute observations of focal birds, found differences between the time spent feeding by Divers of different social/age classes. Adults feeding with juveniles spent less time feeding than the juveniles they accompanied, and solitary juveniles spent the most time feeding (Byrkjedal 2011). McIntyre (1978) found that all birds in the study area fed in the early morning, mid-morning and mid-afternoon, and that they showed a final burst of feeding intensity in the late afternoon and stopped feeding around sunset (occasionally earlier) before rafting 10–20 minutes after sundown.

Divers ingest most of their prey underwater (Barr 1966; King 1976; Evers et al. 2020), with larger prey items that are difficult to swallow being brought to the surface. Observing their head movements can help distinguish prey items: crustaceans are flailed against the water surface to dismember them before swallowing the carapace (King 1976; Evers et al. 2020), while large flatfish (Pleuronectiformes) are struck with powerful bill-open stabs before being swallowed (D. C. Jardine pers. obs.). After ingesting large prey items, Divers normally take one or more short drinks, presumably to assist swallowing, and occasionally they will stand and flap their wings, which also has the potential to assist swallowing (Byrkjedal 2011).

Identifying Diver prey items can be difficult when most are consumed underwater. The stomach contents of 38 adult Divers (20 male and 18 females) were sampled by Collinge (1924) in his study on economic ornithology, and while his account provides quantitative monthly information, it does not provide information of the geographical source of the samples. Overall, Collinge (1924) found fish remains in 32 Diver stomachs (84%) and identified species including gurnard Triglidae sp., Haddock Melanogrammus aeglefinus, Herring Clupea harengus, Sprat Sprattus sprattus, Whiting Merlangius merlangus, sandeels Ammodytes sp., Trout Salmo trutta and flatfish. Collinge (1924) adjudged that 55.3% of Diver prey was fish, 24% was crustacea and annelids, 18.5% marine molluscs, 2.1% unidentified animal matter and 0.1% algae. Furthermore, 55.3% of fish eaten were prey species of commercial interest, with the remaining 44.7% of the diet was neutral to the interests of humans. Collinge (1924) did not indicate the size of the prey items found in the Divers' stomachs. More modern studies of diet from necropsied Divers indicate that Shore Crab Carcinus maenas, Squat Lobster Galathea squamifera, unidentified molluscs, and Whiting or Haddock Melanogrammus aeglefinus (as determined from otoliths) form part of their diet (Heubeck et al. 1993), while Weir et al. (1997) found bivalve species and Norway Lobster Nephrops norvegicus within the stomachs of dead Divers. Leopold et al. (2000) found, in a drowned but apparently healthy second calendar year individual recovered in The Netherlands, remains of gobies (mainly Sand Gobies Pomatoschistus minutus, lengths: 3.5–7.3 cm; weights: 0.4–3.7 g), one Brill Scophthamlus rhombus (8.5 cm; 8.45 g), five 0-group Flounders Platichthys flesus (5.9–8.5 cm; 2.1–8.5 g) and four Shrimp Crangon crangon (0.5 g). See Appendix 1 for details of all prey recorded in the United Kingdom and Ireland.

While the studies listed above provide some insight into the foraging ecology of Divers, there is little contemporary evidence of this important aspect of their ecology in their wintering stronghold in Western Scotland. Therefore this study aimed to investigate the composition of their diet and the factors which influenced their feeding patterns. This evidence will inform assessment of developments that may impact on their wintering foraging habitats and the conservation management of protected areas for Divers.

This study, while considered for many years, would never have commenced without the support and encouragement of Bob Furness and Martin Heubeck, whose comments, along with those of the editors, on an early draft also greatly improved this paper. Bob McGowan (formerly National Museum Scotland), who provided access to the Museum's collections, and Mike Harris provided further encouragement. Karen Boswarva, Patrick Cavanagh and Jack Waldie kindly provided assistance on identification of fish prey, while Clare, Frank and Olive Cavanagh assisted in the measurement of Shore Crabs. Hannah Woods provided statistical advice and Ian Andrews prepared Figure 1. I thank them all.

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• Appendices [PDF]