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moult (M. Harris, unpublished data), though this has only been observed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.