In recent years, the development of remote sensing technologies to study wildlife has changed the way we can understand certain ecosystems that are inaccessible for scientists due to their inhospitable characteristics. The use of high-resolution satellite imagery is a new tool that made it possible to study the populations of the iconic emperor penguin (Aptenodytes forsteri).
Those images, which range from 30 to 60 cm in resolution, made it possible to distinguish these seabirds during their breeding season in sea-ice along the coast of Antarctica. Their detection is because these seabirds, usually in groups of hundreds or even thousands of individuals, leave a very representative and big mark of guano (i.e., bird excrements), which can be identified in satellite images by the contrast between the brown color of guano and the white color of the sea-ice. This allowed counting the number of emperor penguin colonies around Antarctica for the first time. Currently, it is known that there are 66 colonies.
But, as perfection is difficult to achieve, all technologies end up having their limitations. To date, to represent the total number of individuals in a given colony, only one satellite image is used per year. But can this really be accomplished with just one image per year? Does this image convey the true dynamics of that colony in one year? These questions that led north-American researchers and international colleagues to study the limitations that this technology may have for counting emperor penguins.
In this study, the researchers evaluated two types of variables that can influence the penguin counts in a given image. These variables related to the satellite itself (e.g., solar azimuth), which can cast shadows on the image preventing the correct counting of birds, and environmental variables (e.g., wind and temperature), which can influence whether the dispersion of penguins (more disperse penguins are easier to count).
Three of the most representative colonies in terms of numbers were used as study sites: Atka Bay and Stancomb-Wills (colonies in the Weddell Sea sector) and Coulman Island (a colony in the Ross Sea sector).
The authors concluded that using only one image per year to obtain conclusions about the numbers of emperor penguins in each colony may not be the best way, since the influence of the variables studied led to very different numbers of individuals throughout the months. In future research, these variables should be used as they improve the count of emperor penguins to obtain numbers closer to the reality.
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Source: Labrousse, S., Iles, D., Viollat, L., Fretwell, P., Trathan, P. N., Zitterbart, D. P., Jenouvrier, S. & LaRue, M. (2022). Quantifying the causes and consequences of variation in satellite‐derived population indices: a case study of emperor penguins. Remote Sensing in Ecology and Conservation, 8(2), 151-165. https://doi.org/10.1002/rse2.233
Author: Hugo Guímaro