Introduction
For animals operating in aerial and aquatic environments, movement costs
and capacities are profoundly affected by air and water currents
respectively1. This drives widespread and large-scale
patterns of animal movement; with birds selecting tailwinds or updrafts
as they migrate across ocean basins or between land
masses2, and fish minimising counter-flows as they
migrate upstream3. In fish, current selection is also
a key driver of habitat use outside periods of
travel4. Indeed, there is a rich history of research
on fish space-use in relation to flow characteristics in rivers dating
back to the 1960s5, which has demonstrated that water
velocity, and in some cases velocity shear, determine abundance, habitat
use (both within and between species), and interspecific competition in
some species4. In contrast, there is little to no
research on how local airflows affect the distribution of birds outside
travelling, including the distribution of breeding colonies.
Reproductive success is closely linked to the physical characteristics
of breeding sites in many taxa6,7 which can feed into
broader species distribution models8. In colonial
animals, breeding sites can represent the nexus of reproductive activity
for tens of thousands of individuals9. There is
therefore a clear need to establish what drives colony location in these
taxa, in order to identify the availability of breeding habitat, and
predict how areas differ in quality, now and in the
future10-16. Over 95% of seabirds are colonial
breeders17. Seabirds are also more at risk than other
comparable groups of birds, with widespread decline in populations due
to commercial fisheries, pollution, habitat change and the introduction
of invasive predators18. In some cases this has led to
entire breeding colonies being lost19,20. Here,
conservation practitioners need to know where to focus restoration
efforts e.g. by decoy deployment and acoustic attraction to re-seed
breeding activity21. This is crucial given that there
will always be a fitness cost associated with breeding in sub-optimal
habitat22. However, while a wide range of studies have
analysed breeding site characteristics in
seabirds6,7,9,23, and compared them with available
habitat24, we are unaware of any that have
successfully applied predictions from one site to another (cf .25).
The tendency of seabirds to breed on offshore islands and/ or coastal
cliffs has been attributed to the need to reduce exposure to terrestrial
predators and be close to feeding areas9,26.
Nonetheless, for cliff nesting species, it is clear that not all cliffs
are equal, as colonies tend to be clumped, with great swathes of cliff
habitat left empty7. Indeed, cliffs should vary in
their accessibility to terrestrial predators (primarily through
variation in slope angle), as well as the availability of suitable
breeding ledges, with species varying in their need for different ledge
characteristics according to their body size and nest building
habit6,27.
There are also compelling reasons why flow characteristics should affect
breeding habitat preferences, particularly for groups such as seabirds,
which are exposed to strong flows. Wind can affect the risk of eggs/
birds being displaced from the nest28, as well as
influencing exposure to rain (particularly in cliff nesting species) and
heat stress (through evaporative heat loss), both of which can cause
mortality 28,29,30. Wind also has a strong influence
on flight capacity. In common guillemots (Uria aalge ) and
razorbills (Alca torda ), 60% of attempts to land at their cliff
nests were found to fail in a strong breeze31,
suggesting there are advantages to breeding on more sheltered cliffs.
Indeed, frigatebirds (Fregata magnificens ) nest in relatively
wind still areas, despite the associated reduction in ability of birds
to lose heat, which may reflect the difficulties that adults would
experience in remaining on the nest and operating close to it in high
winds due to their low weight and wing loading32. The
importance of being able to maintain flight control close to the nest
suggests that habitat selection could be influenced by several airflow
characteristics, including the strength of the horizontal and vertical
components, as well as the turbulence.
Despite the potential importance of airflows for these animals that
breed in exposed locations, there is a complete lack of information on
the flow characteristics associated with colony presence and absence
(though see33). This is likely due to the difficulties
of quantifying wind over complex, often steep terrain. It may also
reflect our inability to see flow characteristics, in contrast to rivers
where this can be evident from surface characteristics. Where the impact
of wind has been assessed by proxy, wind fetch was found to have
contrasting effects in the presence/absence of colonies of three
Pygoscelis penguin species34, while colony aspect was
not significant for species of the auks family23,31.
Nonetheless, aspect may be a poor proxy for the precise wind conditions
experienced at colonies, as airflow characteristics will be modified by
the particular topography of the surrounding area. As a result, two
cliffs with the same aspect and prevailing wind conditions can
experience very different flow regimes31.
We use computational fluid dynamics (CFD) to provide the first
assessment of whether and how local airflow conditions predicts the
distribution of seabirds, taking colonies of common guillemots
(Uria aalge ) breeding on Skomer island, UK, as our study system.
We estimate a number of airflow characteristics, including the magnitude
of the wind, the horizontal and vertical wind components, air pressure
(as a predictor of exposure) and finally gustiness and turbulence, which
may affect flight control close to the breeding
cliffs31. Our specific objectives were to: (i) assess
whether airflows associated with the prevailing wind direction predict
breeding site selection (patterns of presence and absence), and habitat
quality (colony density), (ii) quantify the airflow conditions that
birds will be exposed to with changes in wind direction and (iii) test
our model of habitat selection by predicting colony presence and absence
on a neighbouring island. This test that is considerably stronger than
standard cross-validation, but rarely performed25.
Overall, our approach should provide insight into the conditions birds
select and avoid in the prevailing wind, and the “penalty” they suffer
in terms of the adverse conditions they are exposed to if the wind
direction changes, either over the short-term, or as part of larger
scale climatic shifts35,36.