How to use (and not to use) movement‐based indices for quantifying foraging behaviour

Abstract

Movement‐based indices such as moves per minute (MPM) and proportion time moving (PTM) are common methodologies to quantify foraging behaviour. We explore fundamental drawbacks of these indices that question the ways scientists have been using them and propose new solutions.To do so, we combined analytical and simulation models with lizards foraging data at the individual and species levels.We found that the maximal value of MPM is constrained by the minimal durations of moves and stops. As a result, foragers that rarely move and those that rarely stop are bounded to similar low MPM values. This implies that (1) MPM has very little meaning when used alone, (2) MPM and PTM are interdependent, and (3) certain areas in the MPM‐PTM plane cannot be occupied. We also found that MPM suffers from inaccuracy and imprecision.We introduced a new bias correction formula for already published MPM data, and a novel index of changes per minute (CPM) that uses the frequency of changes between move and stop bouts. CPM is very similar to MPM, but does not suffer from bias. Finally, we suggested a new foraging plane of average move and average stop durations. We hope that our guidelines of how to use (and not to use) movement‐based indices will add rigor to the study of animals’ foraging behaviour.

Observation resolution critically influences movement-based foraging indices

“…When using the highest resolution, our estimate of CPM (averaged over the individuals and equivalent to MPM≈28.9) was an order of magnitude greater than the highest MPM estimate for lizards reported thus far (i.e., Psammodromus hispanicus, MPM = 4.71 24 ; see Halperin et al . 10 for comparison).…”

“…For example, the foraging behavior of approximately 170 lizard species has been characterized using PTM and MPM (see Halperin et al . 10 for a detailed account).…”

“…To characterize the movement frequency, we followed the recommendation of Halperin et al . 10 , and used Changes Per Minute (CPM) instead of the classic MPM. This frequency index is very similar to MPM but does not suffer from inherent bias and inflated error that are typical of MPM (see Halperin et al .…”

“…This frequency index is very similar to MPM but does not suffer from inherent bias and inflated error that are typical of MPM (see Halperin et al . 10 for details). CPM is calculated by dividing the number of initiations of stops and moves by the total observation duration (minus one time unit, since no changes can be observed after it).…”

“…Lizards: Reilly, McBrayer & Miles 2007;Halperin, Carmel & Hawlena 2017;Fish: Radabaugh 1989;Davis, Spencer & Ottmar 2006;Fu et al 2009;Snakes: Hansknecht & Burghardt 2010;Insects: Ferris & Rudolf 2007;Mundahl & Mundahl 2015). For example, the foraging behaviour of approximately 170 lizard species has been characterized using PTM and MPM (see Halperin, Kalyuzhny & Hawlena (2018) for a detailed account). Huey & Pianka (1981) were the first to introduce PTM and MPM.…”