Population‐wide body mass increment at stopover sites is an unreliable indicator of refuelling rates in migrating waders

Abstract

Estimates of refuelling rates in migrating waders are best based on intra‐seasonal recaptures of individually marked birds. This method, however, has methodological problems associated with capture effects and difficulties in attaining sufficient sample sizes. An alternative method had been proposed whereby refuelling rates are approximated by the body mass increment from the slope of the regression between body masses of all birds caught at a site and date. We investigated the appropriateness of this indicator with a simulation study in non‐synchronized migratory species (i.e. arrivals and departures of individuals at the stopover site are not synchronized). Simulation results indicated that the mass increment across the population may be used as an approximation of refuelling rate only in populations with low turnover rates (percentage of birds arriving at/departing from stopover site per day <2%) and ideally with constant numbers of staging birds. The mass increment of non‐synchronized populations with moderate or high turnover rates (higher than 5%) depends mainly on body masses of arriving birds and is not indicative of the individual rate of refuelling. The results of the simulation study were confirmed with empirical data gathered from Wood Sandpiper Tringa glareola and Common Snipe Gallinago gallinago during their autumn migration at a stopover site in Poland. The population mass increment methods considerably underestimated refuelling rate obtained from the recapture‐based approach of individual birds in both species. As a consequence, we suggest that population mass increment should not be used as an indicator of refuelling rates in non‐synchronized stopover populations of migrating waders.