title: Coupling instantaneous energy-budget models and behavioural mode analysis to estimate optimal foraging strategy: an example with wandering albatrosses.
abstract: BACKGROUND
  How foragers move across the landscape to search for resources and obtain energy is a central issue in ecology. Direct energetic quantification of animal movements allows for testing optimal foraging theory predictions which assumes that animals forage so as to maximise net energy gain. Thanks to biologging advances, we coupled instantaneous energy-budget models and behavioural mode analysis to test optimal foraging theory predictions on wandering albatross Diomedea exulans during the brooding period. Specifically, the instantaneous energy-budget model considered the energetic balance (i.e., the difference between empirical energy gain data and modelled energy expenditure via heart rate values) along the trajectory of a given individual. Four stereotypic instantaneous behavioural modes were identified based on trajectory properties (e.g., speed and turning angle) by applying a new algorithm called Expectation Maximization Binary Clustering. Previous studies on this species have shown that foraging-in-flight is the optimal foraging strategy during the incubation period when albatrosses undertake long-distance movements but no specific foraging strategy has been determined for shorter foraging movements (e.g., brooding period).
  
  
  RESULTS
  The output of our energy-budget model (measured as net energy gain) highlighted the potential optimality of alternative search strategies (e.g., sit-and-wait) during brooding, when birds may be subjected to specific energetic trade-offs and have to adapt their foraging strategies accordingly. However, not all birds showed this pattern, revealing the importance of considering individual variability in foraging strategies, as well as any switching among strategies, before drawing population-level generalizations. Finally, our study unveils the importance of considering fine scale activities to make realistic estimates of trip energy expenditure for flying birds at sea.
  
  
  CONCLUSIONS
  The up-scaling of accurately measured fine-scale energy patterns is essential to quantify energy balances, and their fluctuations by season of different activities among individuals or populations. In particular, we offer new insights for the energetic quantification of the effect of changing oceanic winds on the biology of pelagic predators in the southern oceans.
authors.affiliation: UFZ-Helmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany ; Centre d'Etudes Biologiques de Chizé, UMR 7372 CNRS - Université de la Rochelle, 79369 Villiers en Bois, France ; Instituto Español de Oceanografía, CO Xixón, Camín de l'Arbeyal s/n, Xixón, 33212 Spain.
authors.forename: Maite
authors.initials: M
authors.lastname: Louzao
authors.affiliation: UFZ-Helmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Wiegand
authors.affiliation: ICREA-Movement Ecology Laboratory (CEAB-CSIC), Accés Cala St. Francesc 14, 17300 Blanes, Spain ; CREAF, Cerdanyola del Vallès, Barcelona, 08193 Spain.
authors.forename: Frederic
authors.initials: F
authors.lastname: Bartumeus
authors.affiliation: Centre d'Etudes Biologiques de Chizé, UMR 7372 CNRS - Université de la Rochelle, 79369 Villiers en Bois, France.
authors.forename: Henri
authors.initials: H
authors.lastname: Weimerskirch
keywords: Behavioural clustering; Energy-budget model; Oceanic predator; Oceanic winds; Optimal foraging theory