Consistency among plot-based and plotless methods for Magellanic Penguin density estimations in Tierra del Fuego

Abstract

Density estimations are not independent from the methodology used. Factors than caninfluence density estimation in seabirds include spatial arrangement, colony dynamicsand history over time. As burrow nesting seabirds, Spheniscus penguins´ numbers arenot easily assessed with methods such as aerial surveys. There are at least five methodsbeing used for estimating density and abundance of Magellanic penguins (Spheniscusmagellanicus), with no current consensus on which is more reliable. Moreover, to datethere is no comparison among methods for a single study area. During the breedingseason of 2021, we compared the level of agreement between two methods for densityestimation of Magellanic penguins in Isla Martillo (Argentina), a 44.8 ha island in theBeagle Channel. We compared a plot-based method (i.e., total counts in 20 m radiusplots) and a plotless method (i.e., the point-centered quarter method) and assesseddensity and abundance estimations in three previously established strata and for theentire area. Methods were consistent among each other (i.e., confidence intervals ofdensity estimations widely overlapped). Current stratification has no effect on the totalabundance estimation. Point-centered quarter was a less time-consuming method, butdensity accuracy was sacrificed in the process. Truncation of the data up to 20 m in thismethod also had no effect on density estimation. In future studies and in order toimprove the quality of information for species breeding in very different environmentsthroughout their range, we recommend assessing the consistency between at least twomethodologies, with their associated field efforts.