Results
Foraging niche
transferability
Multi-colony models were not able to successfully generalise the
foraging niche of modelled species across all colonies, showing
generally poor global transferability (mean AUC: 0.56, range 0.4-0.82;
Table 3). However, leave-group-out cross validation showed that
multi-colony models could predict suitable foraging habitat at some
individual colonies (Fig 3).
Exploration of inter-colony transferability revealed that
colony-specific models were transferable to one, or sometimes several,
other colonies, but not all colonies (Fig. 3). For many modelled species
this issue manifested as clustering in colony transferability: groups of
colonies that could predict to one another, but poorly to other groups
of colonies (Fig. 3-J), and partially explains low global
transferability of colony-specific models (Table 3). Inter-colony
transferability was not explained by geographical distance or
oceanographic similarity between colonies (19 of 20 Mantel tests p
> 0.05; supporting information). However, with the
exception of noddies, all multi-colony models had slightly better global
transferability than the colony-specific model average (Table 3),
indicating multi-colony models generalised information from disparate
colony clusters, boosting transferability.
We found that multi-colony models frequently (species mean: 41%; Table
3) matched or outperformed colony-specific models at local prediction
(multi-colony leave-group-out cross validation vs colony-specific
spatial cross validation; multi-colony row values vs diagonal values in
Fig. 3). This finding indicates that the global foraging niche was able
to predict suitable foraging habitat at approximately 40% of colonies,
with comparable or better accuracy than models of the local foraging
niche, although this was strongly species dependent (0% for noddies,
66% for frigatebirds and terns; Table 3). However, all of the
colony-specific models from GBR tracking were superior to their
multi-colony model equivalent (brown booby from Raine Island and Swain
Reefs, Fig. 3A; masked booby from Swain Reefs, Fig. 3E; wedge-tailed
shearwaters from Heron Island on short trips, Fig. 3I and long trips,
Fig. 3G; and noddies from Heron Island, Fig. 3C).
Despite poor overall global transferability, we observed differences
between modelled species (Table 3). Booby species had similar
multi-colony global transferability (AUC: 0.53-0.55) and showed
clustering of inter-colony predictive performance (Fig. 3A, E, F). Brown
and masked booby colony-specific models had a greater range of
transferability than red-footed booby but the same overall average (AUC:
0.51). Frigatebirds and wedge-tailed shearwater short trip multi-colony
models (AUC: 0.61 and 0.58, respectively) showed better global
transferability than boobies, and the greatest transferability increase
over the colony-specific model average. Both of these modelled species
showed clustering of inter-colony predictive performance, the former
demonstrating examples of good inter-species and inter-colony
transferability (Fig. 3G, I). Tropicbirds and wedge-tailed shearwater
long trip multi-colony models had poor global transferability,
equivalent to that of boobies (AUC: 0.56 and 0.54 respectively). Sooty
terns and noddies showed the poorest global transferability (AUC: 0.48
and 0.40 respectively), with the latter’s multi-colony model being
outperformed by the colony-specific model average. Terns showed the best
global transferability between colonies (AUC: 0.82) but were limited by
sample size (Fig. 3B).
Table 3 Summary of model transferability and foraging radii. Global
transferability is the predictive performance of a model averaged over
all colonies (MEAN column; Fig. 3). Local prediction gives the
percentage of colonies where the multi-colony model matched or
outperformed self-prediction by the colony-specific model. Foraging
ranges are summarised across tracked colonies (for colony breakdown see
supporting information), and inclusion of known foraging areas in global
foraging circles are averaged across all tracked colonies.