Unreliable inferences about chinstrap penguin population trends: a statistical critique and reanalysis

Chinstrap penguin (Pygoscelis antarctica) populations have declined at monitored sites in the Western Antarctic Peninsula since (at least) the 1980s. Recent efforts, such as the Mapping Application for Penguin Populations and Projected Dynamics (MAPPPD) have increased and improved the availability of data on penguin abundance, including chinstrap penguin population counts, throughout the Antarctic Peninsula region. However, historical count data are sparse and sporadic, and population trends cannot be reliably assessed at many chinstrap penguin colonies. Aiming to fill this gap, Krüger (2023) used available MAPPPD data to predict chinstrap penguin breeding colony trends between 1960 and 2020, to estimate whether the level of population change within three generations exceeded IUCN Red List Criteria. While we applaud the author’s intention and agree that chinstrap population trends are an important area of research, we caution that the analysis performed by Krüger cannot support robust inference. Thus, even though it is possible that the regional chinstrap penguin population in the Antarctic Peninsula may have declined by ~30 % within three generations, this claim cannot legitimately be made based on the results presented in Krüger (2023). The aims of this paper are to (1)  highlight data gaps for chinstrap penguin populations; (2) illustrate the analytical shortcomings in Krüger (2023) with  emphasis on the need for model checking; (3) perform a simulation study and reanalysis of MAPPPD data to show that an improved model can yield better predictions of chinstrap population trends, at least within the range of observed data; and (4) encourage the further practice of open science.