Biological findings
We have shown that the flightless moult strategy of breeding puffins varies markedly between individuals, and possibly colonies, despite usually being a fixed life-history trait within migratory bird species (Barta et al. 2008). We also found evidence that some individuals may undergo flightless moult twice in a non-breeding season, with this strategy possibly tied to more extensive migrations, though this relationship is based on a very small sample size. If this is the case, our results provide the first potential evidence for two flightless moult periods per year in a wild volant bird species (Beltran et al. 2018).
The exploration-refinement hypothesis (Guilford et al. 2011) suggests development over time of a fixed migration strategy that exploits predictable prey availability in space and time, leading to inter-individual variation (e.g. Harris et al. 2015). More extensive migration may allow puffins to exploit reliable food resources (Jessopp et al. 2013), especially during moult when their diving abilities are likely compromised (Bridge 2004). Prolonged flight during migratory phases may lead to accelerated feather wear and reduced flight efficiency for a bird with an already high wing-loading (Navarro & González-Solís, 2007, Greenewalt 1975), whose burrow nesting habits likely accelerate flight feather wear during the breeding season. Increased energy requirements for long-distance migrants also requires increased foraging effort and dive rates (Fayet et al. 2016), potentially further wearing wing feathers. This may result in two moults in one non-breeding season to maintain flight feather condition and retain flight efficiency (Barta et al. 2008). It may be that one or both moults are incomplete, allowing the puffins to remain partially volant. Small alcids in the genus Aethia forego synchronous moult, instead staging the replacement of primary flight feathers, allowing them to continue flying throughout moult (e.g. Bond et al. 2013). Some storm-wrecked puffins have shown evidence of a similar partial primary moult (M. Harris, unpublished data), though this has only been observed in a very small proportion of a very large sample of recovered birds, so is likely an anomaly or due to poor health. Biannual synchronous flightless moults have been observed in captive juvenile puffins (Swennen 1977, M. Huwiler, Tierpark Bern, pers. comm.), and while it is uncertain how these observations relate to wild breeding adults (Thompson & Kitaysky, 2004), it does highlight that this strategy is physiologically possible. In contrast, the two Skomer individuals that stayed closer to the colony (< 1500km) during the non-breeding season clearly underwent a single flightless moult in autumn, not long after the summer breeding season. Flight feather moult is energetically demanding (Guillemette et al. 2007) and reduces foraging efficiency (Bridge 2004), so there are potential advantages in strategies that forego a second flight feather moult where possible. A trade-off likely exists between the energy required to undergo long-distance migration to highly productive areas, potentially necessitating two flightless moults, versus reduced migration effort and a single flightless moult, while also remaining in an area where feeding conditions may be poorer. Put simply, moult strategy in puffins could be dichotomous (biannual versus annual moult) associated with high versus low energy intake and expenditure, reflected in the activity budgets of long- and short-distance migrants (Fayet et al. 2017).
Previous studies, based on birds recovered dead rather than those from birds equipped with loggers that survived the non-breeding season, described an early/late bimodal distribution of puffin moult timings in the North Sea and around the Faroes Islands, with peaks in October and March (Harris et al. 2014). This timing largely agrees with our findings from Skomer individuals. It may be that dead birds identified as moulting in March were going through a second moult, though many more suitable tracking data, e.g. from dual-equipped puffins, would be required to suggest this with any confidence. The inferred moult of three Skellig Michael individuals occurred once, from December to February, with no evidence that this follows an earlier post-breeding flightless moult, although again, this is based on a small sample where moult could be resolved from single logger data streams. Similar moult timings were observed by Anker-Nilssen et al. (2018), who found that most puffins found following storm wrecks on the coast of Norway in February/March 2016, likely originating from colonies on the east coast of the UK, were in the latter stages of moult and had only recently become volant. Birds found dead are more likely to have been wintering relatively close to land, and so may not provide an unbiased sample of the wider population (Fayet et al. 2017). It is also possible that storms disproportionately affect moulting puffins that cannot fly to escape storm tracks, with reduced foraging efficiency during moult further compromised by storm conditions (Clairbaux et al. 2021). This does not seem to universally be the case, with a high proportion of moulting birds found in one wreck on the Norwegian coast (Anker-Nilssen et al. 2018) and a low proportion in another in the Bay of Biscay (Morley et al. 2017), despite both wrecks occurring at a similar time of year.