Timing of reproduction is a critical life-history trait and implies major fitness consequences. Differences in life history strategies can lead to different responses to the same environmental conditions even among closely related species. Investigating the factors affecting the timing of reproduction in closely related and sympatric species may help understanding the relationship between different life history traits and species-specific responses to ecological factors. We investigated the effects of season (photoperiod), local climatic variability and prey abundance on the breeding season of two sympatric insectivorous birds that differ in the timing of reproduction and moult and have different migration strategies: the Moustached Warbler Acrocephalus melanopogon and the Eurasian Reed-warbler Acrocephalus scirpaceus. During a five-year period, breeding phenology was determined by examining bird incubation patches during standardised captures and food availability was assessed by invertebrate sampling. Time, mean temperatures and the abundance of some prey taxa were inter-correlated and all contributed to influence the breeding phenology of both species according to partial least squares regression analysis. Besides some interspecific differences in the effect of environmental factors on the breeding phenology, we found strong similarities between the patterns observed in the two warblers. This suggests that, in spite of interspecific differences in life history, the ecological mechanisms shaping reproduction phenology are similar in the two warblers and the same environmental factors are involved in determining the timing of their reproduction.—Ceresa, F., Belda, E.J., Brambilla, M., Gómez, J., Mompó, C. & Monrós, J.S. (2020). Factors shaping breeding phenology in birds: an assessment of two sympatric Acrocephalus warblers with different life histories. Ardeola, 67: 371-385.