How important is it to standardise the measured mass of shorebirds weighed at varying intervals after capture?

Shorebirds are often caught in large samples and kept in temporary captivity for differing lengths of time before being banded, measured, weighed and released. As birds in temporary captivity lose mass as they digest their food and void their guts, those weighed first may have a higher relative mass than those weighed last, which can affect the mean recorded mass of a large catch. We recorded multiple mass measurements of individual shorebirds of four species Red Knot Calidris canutus (mean mass 148 g), Ruddy Turnstone Arenaria interpres (135 g), Dunlin C. alpina (64 g) and Semipalmated Sandpiper C. pusilla (29 g), caught in a series of catches on Delaware Bay, USA during spring passage. We modelled the rate of mass loss to provide a correction factor allowing individuals to be compared across catches and the mean mass of all birds in different catches to be compared. As birds caught in cannon-nets are rarely measured until at least 30 minutes after capture, masses were predicted for this time, thus reducing the size of the correction required. Time since capture was the most important predictor of mass loss. The mean defecation rate (number of droppings per bird per minute) was closely related to mass loss. Correction of mass for a catch of 372 Red Knot changed the mean mass of the birds in the catch from 147.8 g to 151.0 g, a difference of 3.2 g or 2%, although this was not significantly different from zero. To compare mass between catches, or to estimate fitness for departure, researchers should consider whether mass should be corrected to a standard time, taking account of local conditions such as when the birds last fed, the time since capture at which the first birds are weighed, and the maximum time between first and last weighing.