title: Pushed for time or saving on fuel: fine-scale energy budgets shed light on currencies in a diving bird.
abstract: Animals may forage using different currencies depending on whether time minimization or energy maximization is more pertinent at the time. Assessment of net energy acquisition requires detailed information on instantaneous activity-specific power use, which varies according to animal performance, being influenced, for example, by speed and prey loading, and which has not been measured before in wild animals. We used a new proxy for instantaneous energy expenditure (overall dynamic body acceleration), to quantify foraging effort in a model species, the imperial shag Phalacrocorax atriceps, during diving. Power costs varied nonlinearly with depth exploited owing to depth-related buoyancy. Consequently, solutions for maximizing the gross rate of gain and energetic efficiency differed for dives to any given depth. Dive effort in free-ranging imperial shags measured during the breeding season was consistent with a strategy to maximize the gross rate of energy gain. We suggest that the divergence of time and energy costs with dive depth has implications for the measurement of dive efficiency across diverse diving taxa.
authors.affiliation: Department of Pure and Applied Ecology, Institute of Environmental Sustainability, University of Swansea, Swansea SA2 8PP, UK. s.l.c.shepard.381601@swansea.ac.uk
authors.forename: Emily L C
authors.initials: EL
authors.lastname: Shepard
authors.forename: Rory P
authors.initials: RP
authors.lastname: Wilson
authors.forename: Flavio
authors.initials: F
authors.lastname: Quintana
authors.forename: Agustina Gómez
authors.initials: AG
authors.lastname: Laich
authors.forename: Dan W
authors.initials: DW
authors.lastname: Forman
mesh_terms: D000818:Animals; D001717:Birds / Q000378:metabolism / Q000502:physiology*; D001947:Breeding; D004734:Energy Metabolism; D005426:Flight, Animal; D011235:Predatory Behavior; D012621:Seasons; D013997:Time Factors