Recent climate warming has resulted in advancement in laying dates and changes in various bird breeding parameters. The study of the breeding biology of Great Reed Warblers Acrocephalus arundinaceus was carried out at Milicz fishponds (SW Poland) during 18 discontinuous breeding seasons between 1970 and 2017. Over the study period, Great Reed Warblers started breeding progressively earlier; both the earliest and median first egg laying dates advanced, whereas the latest laying dates remained unchanged. Egg volume and clutch size did not change between 1972 and 2017. Mean clutch size of all nesting attempts was 4.8 (SD = 0.69, n = 1135). Nestling growth rates were similar in 1971 and 2016. Nestling period shortened during the study period. The decrease amounted to 0.7 days between 1970–1984 and 2004–2017. No temporal trends in fledgling production were observed. The proportion of successful broods varied considerably between years (range: 37%−76%) but did not show any consistent trend throughout the study period. More cases of Cuckoo Cuculus canorus parasitism were observed in 1997−2017 than in 1970−1984 (7% of nest losses versus 0 respectively), whereas starvation of nestlings was more frequent in 1970−1984 (10% versus 2% of nest losses). Breeding densities varied between seasons with no significant trend across the study years. This study concludes that the advanced start of breeding has not significantly influenced basic parameters of breeding ecology of the Great Reed Warbler.