Diurnal patterns of body mass change during stopover in a migrating songbird, the northern wheatear Oenanthe oenanthe

In migrating birds body mass change during stopover is often used to evaluate the quality of a stopover site. Because such body mass changes are difficult to survey in migrating birds various methods were developed to allow the analysis of larger sample sizes. In this article we present patterns of repeated body mass measurements of individual birds and the commonly used method of plotting body mass over time of day of birds being trapped only once. We repeatedly measured body mass of 89 northern wheatears Oenanthe oenanthe, when stopping over at a small island in the North Sea. A balance beneath bowls offering mealworms ad libitum was used to weigh their body mass several times per day. From these repeated body mass measurements we have generated a general model of daily mass gain patterns and nocturnal mass loss. Daily body mass changes followed in general an asymptotic curve progression with highest gain rates in the morning hours. During night birds lost about 5% of the evening body mass irrespectively of local wind force, temperature, night length or precipitation. By plotting first traps by time of day no such pattern in body mass increase was evident. Even in a simulated situation with repeatedly measured birds increasing their body mass, no such body mass increase could be shown when plotting one randomly chosen measurement of each bird per day. This first-traps-by-time-of-day-method depends highly on sample size, overall mass increase and the mass variation between individuals and will produce a traceable body mass increase only under certain circumstances.