Acceleration as a proxy for energy expenditure in a facultative‐soaring bird: Comparing dynamic body acceleration and time‐energy budgets to heart rate

Abstract

Abstract In animal ecology, energy expenditure is used for assessing the consequences of different behavioural strategies, life‐history events or environments. Animals can also influence energy expenditure through instantaneous behavioural responses to their external environment. It is therefore of interest to measure energy expenditure of free‐ranging animals across seasons and at high temporal resolutions. Heart rate has historically been used for this, but requires invasive surgery for long‐term use. Dynamic body acceleration (DBA) is an alternative proxy for energy expenditure that is simpler to deploy, yet few studies have examined how it performs over extended time periods, or for species using different locomotory modes, especially passive modes like soaring flight. We measured DBA alongside heart rate in free‐ranging lesser black‐backed gulls, a seabird that moves using flapping flight, soaring and walking, and rests on both land and water. Our objectives were to compare the relative changes in DBA and heart rate among and within behaviours and to examine how accelerometers can be used to estimate daily energy expenditure by comparing DBA to time‐energy budgets (TEBs). DBA and heart rate were sampled concurrently at 2.5‐ and 5‐min intervals throughout the breeding season, though measurements were not exactly synchronised. Behaviour was identified from accelerometer measurements, and DBA and heart rate were averaged over bouts of consistent behaviour. Heart rate was converted to metabolic rate using an allometric calibration, after confirming its fit using metabolic measurements taken in captivity and values from existing literature. Both proxies showed similar changes among behaviours, though DBA overestimated costs of floating, likely due to waves. However, relationships between DBA and heart rate were weak within a behaviour mode, possibly due to the lack of synchrony between proxy measurements. On daily scales, DBA and TEBs perform comparably for estimating daily energy expenditure. Accelerometery methods deviated from a 1:1 relationship with heart rate because acceleration could not measure variation in resting metabolic costs. We conclude DBA functions well for detecting energy expenditure arising from activity costs, including during soaring flight. We discuss scenarios where one method (DBA vs. TEBs) may be preferred over the other. Read the free Plain Language Summary for this article on the Journal blog.