1. The seasonal migration of birds is divided into alternating phases of stopover and flight. The fuel deposition rate at stopover sites is the crucial factor determining overall speed of migration and its success. Therefore, field data about the variation in fuel deposition rates at different levels (among sites, seasons, days, individuals) are essential to explain the observed behavioural reactions to environmental variability and migration strategies. 2. Fuel deposition rates of four species of passerine migrant birds captured at 14 stopover sites from northern Europe to sub-Saharan Africa during autumn and spring were analysed. Plasma concentrations of triglycerides and β-hydroxy-butyrate were used to estimate relative fuel deposition rates (named fattening index). 3. The largest variation in fattening indices was between sites which was only weakly explained by geographical position relative to ecological barriers and did not differ between spring and autumn. Therefore it is expected that the selection of appropriate stopover sites is of paramount importance for the success of migration. 4. Variation in fattening index between days within sites was comparatively small with no obvious seasonal trend over the restricted period of mid-migration and with no correlation between subsequent days. However, only samples during good weather days were examined and the full range of this variation could not be estimated. Air temperature and duration of rainfall could explain part of this variation, probably because invertebrate availability is higher during warm and dry weather. 5. The variation of fattening rates between birds on the same day and site was low, and could partially be explained by body mass. Heavy birds had higher fattening rates than light birds. 6. From this analysis, it appears to be difficult for migrating birds to predict the fattening rate of the next stopover site, but relatively easy to assess the fattening rate of the current site in a short time. This may result in larger departure fat loads than would be necessary to reach the next stopover site.