Tracking Ocean Wanders (PDF, 5 MB) - BirdLife International

8
Tracking ocean wanderers: the global distribution of albatrosses and petrels – Methods
Figure 2.8. Utilisation distributions (UDs) and range for breeding Buller’s Albatrosses from Snares Islands. Each map was produced from the
PTT tracks of four individuals selected at random from the complete dataset.
Figure 2.9. Utilisation distributions (UDs) and range for breeding Buller’s Albatrosses from Snares Islands. Each map was produced from the
PTT tracks of 12 individuals selected at random from the complete dataset.
The area of the 50% UD requires fewer tracks to reach a
stable maximum value, which is encouraging for the
purpose of identifying marine Important Bird Areas as
these will be located in areas of high-intensity use. However
care is needed as the locations of these hotspots are not
necessarily the same for each random sample: in Figure 2.8
four samples of the Buller’s dataset are shown, using four
tracks drawn at random from the complete dataset. Even
though the total area of the 50% UD is similar to that of
the complete dataset, different regions are highlighted and
some apparent hotspots are missed completely. At sample
sizes this small, the foraging behaviour of a single
individual on a single trip can produce hotspots in regions
not frequented by any other individuals in the dataset. If
the random sample is increased to 12 the influence of a
single individual is reduced and hotspots are found in
similar areas to the complete dataset (Figure 2.9), although
some are still missed. The possibility of missing hotspots
should be borne in mind when interpreting maps
irrespective of the sample size.
Frances Taylor, Aleks Terauds and David Nicholls