How many seabirds do we need to track to define home-range area?
Soanes, LM and Arnould, JPY and Dodd, SG and Sumner, MD and Green, JA, How many seabirds do we need to track to define home-range area?, Journal of Applied Ecology, 50, (3) pp. 671-679. ISSN 0021-8901 (2013) [Refereed Article]
Copyright 2013 The Authors. Journal of Applied Ecology Copyright 2013 British Ecological Society
1. In recent years, marine predator and seabird tracking studies have become ever more popular.
However, they are often conducted without first considering how many individuals
should be tracked and for how long they should be tracked in order to make reliable predictions
of a population’s home-range area.
2. Home-range area analysis of two seabird-tracking data sets was used to define the area of
active use (where birds spent 100% of their time) and the core foraging area (where birds
spent 50% of their time). Analysis was conducted on the first foraging trip undertaken by the
birds and then the first two, three and four foraging trips combined. Appropriate asymptotic
models were applied to the data, and the calculated home-range areas were plotted as a function
of an increasing number of individuals and trips included in the sample. Data were
extrapolated from these models to predict the area of active use and the core foraging area of
the colonies sampled.
3. Significant variability was found in the home-range area predictions made by analysis of
the first foraging trip and the first four foraging trips combined. For shags, the first foraging
trip predicted a 56% smaller area of active use when compared to the predictions made by
combining the first four foraging trips. For kittiwakes, a 43% smaller area was predicted
when comparing the first foraging trip with the four combined trips.
4. The number of individuals that would be required to predict the home range area of the
colony depends greatly on the number of trips included in the analysis. This analysis
predicted that 39 (confidence interval 29–73) shags and 83 (CI: 109–161) kittiwakes would be
required to predict 95% of the area of active use when the first four foraging trips are
included in the sample compared with 135 (CI 96–156) shags and 248 (164–484) kittiwakes
when only the first trip is included in the analysis.
5. Synthesis and applications. Seabird and marine mammal tracking studies are increasingly
being used to aid the designation of marine conservation zones and to predict important
foraging areas. We suggest that many studies may be underestimating the size of these
foraging areas and that better estimates could be made by considering both the duration and
number of data logger deployments. Researchers intending to draw conclusions from tracking
data should conduct a similar analysis of their data as used in this study to determine the
reliability of their home-range area predictions.
central-place foragers, global positioning system, home-range area, marine protected area