Environmental parameters associated with colony distribution on
Skomer island
Wilcoxon rank sum tests for all colony classifications revealed
statistically different slope angles between colonies and non-colonies
on Skomer Island (taking any positive count of birds as colony presence,
W = 611724994, p-value < 2.2e-16). Unoccupied sections were
generally less steep than occupied sections (median slope angles
45o and 51o respectively) and the
densest and largest colonies were associated with the steepest cliffs
(Fig. 1). Slope angle varied with cliff orientation, with mean slope
angle being lowest for cliffs with S and SE orientations. Interestingly,
cliffs facing SW have relatively high mean slope angles (Fig. 1a).
Despite this, most occupied sections were orientated away from the SW
(Fig. 2a). Both the largest, as well as the densest colonies, were
mainly orientated towards the NW and E.
Models of wind and slope angle significantly predicted colony presence
and absence on Skomer, with the model correctly identifying 80% of the
colonies and 93% of avoided sites for a SW wind, where presence was
taken as the 10 largest colonies (Table 1). In fact, models of colony
location performed well across all modelled wind directions except for
SE winds, indicating that colonies are characterised by particular sets
of flow characteristics in most scenarios. Indeed, in each wind
direction, one colony classification could be predicted by airflow
parameters alone, without slope angle. The lowest model performance was
seen with SE winds where only one colony classification yielded a
reasonable model fit (McFadden41 R2≥ 0.2, table 1). In this model, colony presence was predicted by slope
angle alone (Table 1).
Across the different colony classifications, the pseudo
R2 was lowest overall in models of any occupancy
(1st classification SW 0.28; NW 0.24, SE 0.12, NE
0.38) compared to those predicting the 10 largest colonies
(2nd classification SW 0.59; NW 0.58,SE 0.32, NE 0.74)
or the 11 densest (3rd classification SW 0.17; NW
0.40, SE 0.15, NE 0.67) (Table 1). The overall accuracy and true skill
statistics followed the same general trend, being highest for the 10
largest colonies (Table 1). The sensitivity tended to be somewhat lower
than specificity in the 1st classification
(sensitivity was: SW 0.63; NW 1.00, SE 0.21, NE 0.60 and specificity
was: SW 0.69; NW 0.21, SE 0.96, NE 0.84), but increased in the
2nd and 3rd classifications
indicating a better ability to predict true presence compared to true
absence for the largest and densest colonies.
While mean slope angle was included in two of three models for SW, NW
and NE wind directions, airflow parameters always had a higher effect
size (SI Table 2). A narrow set of airflow parameters was identified as
significant in predicting colony presence across wind directions (Table
1). Furthermore, there was broad agreement between the airflow
characteristics identified within each wind direction, irrespective of
the way colonies were classified. Pressure statistics were included in
all SW models, while TKE and horizontal wind speed parameters were
included in all NW and NE models, respectively.
Unexpectedly, wind speed (in terms of median or skewness) was not
included in the simplest top models of SW winds, instead colonies were
associated with a lower median pressure and pressure gradient (Table 1),
both of which are linked to lower exposure (Fig. 2, Fig. 3).
Colonies were associated with higher turbulence compared to unoccupied
sites, particularly in the case of NW and NE winds (Table 1). In NE
winds, colonies experienced both higher wind speeds (Fig. 4) (positive
horizontal median, 1st classification and negative
horizontal skewness, 2nd and 3rdclassifications) and higher turbulence (here TKE estimates in the
1st and 3rd classifications).