Introduction
Biologging and telemetry studies have greatly advanced our knowledge of the behaviour and distribution of far-ranging animal species (e.g. Jouventin & Weimerskirch 1990, Kooyman 1966). They have also provided insights into their behaviour especially when direct observation is impossible or impractical (e.g. Wilson et al. 1991, Michel et al. 2022), which is often the case for the many marine species that spend prolonged periods at sea far from land (e.g. Weimerskirch et al. 2006, Doyle et al. 2015, Bennison et al. 2019, Brooke 2018). Long-term device attachment allows us to explore distribution and behaviour over broad timescales, such as diel patterns of movement (Seyer et al. 2021), and describe life-history strategies, such as migration (Amélineau et al. 2021) or periodic moult (Grissot et al. 2018). By looking at the behaviour of an animal in relation to its spatial and temporal distribution, it is possible to identify key areas of conservation concern, and identify drivers of declines (Frederiksen et al. 2012, Fayet et al. 2021).
Many of the world’s seabirds are threatened and declining (Dias et al. 2019, Paleczny et al. 2015), creating a pressing need to better understand vulnerable stages of their annual cycle. Seabirds tend to be highly susceptible to the impacts of climate change, including sea temperature rise and shifts in prey distribution and abundance (Durant et al. 2003, Sandvik et al. 2005), as well as extreme weather events (Clairbaux et al. 2021), with large wrecks recorded following severe winter storms at sea (Harris et al. 2014, Morley et al. 2017, Anker-Nilssen et al. 2018). Larger members of the seabird familyAlcidae (hereafter alcids) moult all their primary flight feathers simultaneously (Peery et al. 2008) leading to a protracted flightless period, placing them at greater risk from such dynamic stressors. Alcids are often the most common species washed ashore in storm wrecks in the northern hemisphere (Morley et al. 2017). Obligatory flightless moult places them at greater risk from storm events, as they would not be able to fly to avoid the storm track. Their vulnerability to predation may be increased by their silhouette in downwelling light, which can provide a visual cue to underwater predators (Ulman et al. 2015, Doyle et al. 2015) that they may struggle to escape from when flightless. It may also increase their vulnerability to surface pollutants, because of the increased time spent on the water surface and the inability to escape expansive films of harmful substances such as petroleum oil (Robertson et al. 2012).
The Atlantic puffin (Fratercula arctica ), hereafter puffin, is a seabird species that has undergone rapid population declines across most of their European breeding range during the 2000s (Harris & Wanless 2011), leading to their classification as Endangered in Europe by the European Red List Assessment in 2015 (BirdLife International, 2015). Puffins must carefully time and locate their moult to coincide with sufficient food availability, which can be patchily distributed at sea (Fauchald 2009, Jessopp et al. 2013). Clairbaux et al. (2021) calculated the fasting endurance of puffins as 6.5 (±2.5) days in mid-autumn, and 4.6 (±0.6) days in winter. Local depletion of food during moult therefore puts puffins at risk of starvation. Anker-Nilssen et al. (2018) found that most puffins washed ashore in a post-storm wreck in southwest Norway in early 2016 were in the late stage of primary moult, and almost all individuals were emaciated. Even though many would have been able to fly by the time they were washed ashore, moult may have prevented them from escaping the storm when they were flightless, during which they clearly struggled to find food, despite a highly varied diet (Baillie & Jones 2004), especially during the non-breeding season (Falk et al. 1992, Harris et al. 2015).
The duration, timing, and location of moult in puffins has proven difficult to determine, because it occurs during the non-breeding season when birds are away from their colonies. In other alcids, moult takes place shortly after the breeding season (Gaston & Jones 1998, Peery et al. 2008), facilitating observation of moulting individuals. Harris et al. (2014) assessed moult stage based on feather development of puffins either washed ashore during storm wrecks or shot at sea. They reported primary flight feather moult in puffins at any time from September to March, with peaks in October and March, which is considerably more variable than that of other alcid species that undergo flightless moult (Gaston & Jones 1998). Identifying the flightless period of surviving individuals using biologging studies has so far proven difficult. Leg-mounted saltwater immersion loggers are commonly used to classify seabird behaviour during the non-breeding season. Reduced time spent flying, during flight feather moult for instance, is usually reflected by an elevated proportion of time the leg and logger are wet (Grissot et al. 2019). Puffins, like other alcids, repeatedly tuck their legs into the plumage when on the water (Harris et al. 2010, Linnebjerg et al. 2014, I. Sempere, Oceanário de Lisboa, pers. comms, figure 1), confounding simple behaviour classification using these loggers.
This study uses light-level geolocators with integrated saltwater immersion switches deployed on puffins to identify patterns of behaviour thought to be consistent with flightless moult. By combining data from four individuals with a geolocator on each leg (dual-equipped birds), we developed a behavioural classification method using raw light and saltwater immersion data. We show that we can use results from dual-equipped birds to quantify and correct for behaviours that would confound traditional methods, therefore identify flightless periods assumed to represent moult. We then adapt and validate this method for single-equipped birds, for which there are far more data. This approach may help us to identify overwintering strategies and areas of conservation concern for puffins and other alcids, whose highly restricted mobility during flightless moult may compound the negative impacts of environmental threats.