How many seabirds do we need to track to define home‐range area?

Abstract

Summary1. 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.

Individual-level Variation and Higher-level Interpretations of Space Use in Wide-ranging Species: An Albatross Case Study of Sampling Effects

“…Our approach is similar to previous studies (Wood et al, 2000;Hindell et al, 2003;Taylor et al, 2004;Breed et al, 2006;Soanes et al, 2013;Orben et al, 2015) but our focus is not identifying the presence of an asymptote but rather the range in output at each sample size. For each dataset, we randomly sub-sampled n individuals (without replacement) beginning with n = 3 and increasing in increments of two, up to three less than the maximum number available.…”

“…Grid cell methods offer a simple alternative, where the cumulative time spent within cells of a predefined grid size is used to identify the extent of a group's range and areas of most intense use (e.g., Soanes et al, 2013). Other methods take a habitat preference modeling approach, which takes into account environmental factors that shape patterns of space use (e.g., Aarts et al, 2008;Wakefield et al, 2011;Raymond et al, 2015).…”

“…We are certainly not the first to caution that small sample sizes of biologger-tracked individuals increase the probability of erroneous higher-level conclusions (e.g., Lindberg and Walker, 2007;Hebblewhite and Haydon, 2010;Schofield et al, 2013;Soanes et al, 2013). Further, the examination of intra-population variation among individual movements is presently a burgeoning field in biologging studies of marine vertebrate behavior (reviewed by Patrick et al, 2014).…”

“…The assumption of representativeness is sometimes briefly conceded, but implications for the biological interpretations of the results generally are not formally evaluated. A number of marine vertebrate studies have illustrated an asymptotic saturation effect of increasing the number of tracked individuals or number of foraging trips per individual on estimates of the size of the area occupied by a sample of tracked animals in a pooled analysis (e.g., Wood et al, 2000;Hindell et al, 2003;Taylor et al, 2004;Breed et al, 2006;Soanes et al, 2013;Orben et al, 2015). These studies suggest that a sample of individuals may be representative of their respective group if the estimated occupied areas reach an asymptote before the maximum sample size is included in the analysis.…”

“…In addition to the estimated size of the area occupied by a group, it has also been demonstrated that the geographic locations of contours resulting from pooled analyses of different individuals can vary depending on the degree of individual variation within the sample (Taylor et al, 2004;Breed et al, 2006;Orben et al, 2015). Beyond these few examples which directly address assumptions of group-level representativeness of a sample, consistencies in movements among individuals comprising a dataset and among members of the higher-level group they represent remain un-tested assumptions, especially in seabird studies with small sample sizes (Soanes et al, 2013).…”