Quantifying migration behaviour using net squared displacement approach: Clarifications and caveats

Abstract

Estimating migration parameters of individuals and populations is vital for their conservation and management. Studies on animalmovements and migration often depend upon location data from tracked animals and it is important that such data are appropriately analyzed for reliable estimates of migration and effective management of moving animals. The Net Squared Displacement (NSD) approach for modelling animalmovement is being increasingly used as it can objectively quantify migration characteristics and separate different types of movements from migration. However, the ability of NSD to properly claify the movement patterns of individuals has been criticized and iues related to study design arise with respect to starting locations of the data/animals, data sampling regime and extent of movement of species.We addre the iues raised over NSD using tracking data from 319 moose (Alces alces) in Sweden. Moose is an ideal species to test this approach, as it can be sedentary, nomadic, dispersing or migratory and individuals vary in their extent, timing and duration of migration. We propose a two-step proce of using the NSD approach by first claifyingmovement modes using mean squared displacement (MSD) instead of NSD and then estimating the extent, duration and timing of migration using NSD.We show that the NSD approach is robust to the choice of starting dates except when the start date occurs during themigratory phase. We also show that the starting location of the animal has a marginal influence on the correct quantification of migration characteristics. The number of locations per day (1-48) did not significantly affect the performance of non-linear mixed effects models, which correctly distinguished migration from other movement types, however, high-resolution data had a significant negative influence on estimates for the timing of migrations. The extent of movement, however, had an effect on the claification of movements, and individuals undertaking short- distance migrations can bemisclaified as other movements such as sedentary or nomadic. Our study raises important considerations for designing, analysing and interpreting movement ecology studies, and how these should be determined by the biology of the species and the ecological and conservation questions in focus.