In birds, molt duration is an important parameter that can affect plumage functionality and consequently the fitness of individuals. However, our knowledge about the factors affecting variation in molt speed is scarce, mostly because of the methodological difficulties of studying avian molt. In this study, we used a ptilochronology-based approach to estimate the growth rate at which tail feathers were produced during molt, and hence to shed light on the relationship between molt duration and feather growth rate. For that purpose, we used three different datasets. Firstly, we tested whether the average molt duration of 22 passerine species was correlated with the mean growth rate of their feathers using both conventional and phylogenetically-corrected statistical procedures. Secondly, we explored this same association between captive Great Tits Parus major. And thirdly, we took advantage of the biannual complete molt performed by Willow Warblers Phylloscopus trochilus to examine if the feathers synthesized during their short summer molt grew faster than those produced during their long winter molt. Feather growth rates were negatively correlated with molt duration in all analyses, revealing that molt duration can be estimated from the growth rate of a single feather. However, predictive power was limited by the fact that molt duration is mainly modulated by molt intensity, which seems to be correlated with ecological constraints in our interspecific approach. We also discuss the implications of our results for the evolution of molt duration, and the potential application of ptilochronology in its study.