Are bio-telemetric devices a drag? Effects of external tags on the diving behaviour of great cormorants

Abstract

Externally attached remote-sensing devices used to study animals in their environment are a possible source of disturbance, notably in terms of drag, for diving species. The aim of the present study was to assess the possible effect of device-induced drag on the diving performance of great cormorants Phalacrocorax carbo. Based on wind-tunnel measurements, we assessed the effect of device size on drag and derived a formula to predict how drag changes as a function of both swim speed and device cross-sectional area. Tests on captive cormorants indicated that drag had an effect on the energy expenditure (using dynamic body acceleration as a proxy) during the dive. Wind tunnel-derived drag metrics were combined with data from the literature to construct a model predicting the power consumption of diving cormorants according to device size. Applying the model to dive data from 6 free-living great cormorants (recorded using implanted time-depth recorders) indicated that a device constituting only ca. 3% of the bird's cross-sectional area could cause a 1.7% increase in power consumption when swimming. However, if a bird maintains constant power underwater, e.g. by decreasing foraging speed with increasing drag, this would result in a 7.1% reduction in the distance travelled during the bottom (active hunting) phase of the dive. Device-related increases in drag are also likely to reduce the maximum speeds achievable by these pursuit predators. The present study highlights the interaction between both drag coefficient and swim speed for diving animals with externally attached devices.