Non-invasive measurement of metabolic rates in wild, free-living birds using doubly labelled water

1. Doubly labelled water (DLW) is routinely used to measure energy expenditure and water turnover in free-ranging animals. Standard methods involve capture, blood sampling for baseline measurement, injection with isotopic tracers, captivity for an equilibration period, post-dose blood sampling, release, and subsequent re-capture for final blood sampling. Single sampling methods that minimise disturbance by reducing capture and handling time have been developed and tested. Sampling faeces rather than blood could further reduce disturbance to study animals in a range of species and study systems. However, the extent to which estimates of metabolic rate derived from blood and faecal samples diverge has not been investigated. 2. We compared isotopic enrichment in blood and faecal samples taken concurrently from captive Southern Pied Babblers Turdoides bicolor. Isotopic enrichment levels in faeces and in blood were used to calculate initial and final ratios of (δ^18 O_i)/(δ^2 H_i ) for each ...