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Animals are exposed to the more frequent stressful weather events that may impact parental decisions and their investment in offspring. This study examines whether and how heavy rainfall influences the birds' incubation behavior, hatching date, food volume brought to the nests, the offspring's immune response to phytohaemagglutinin (PHA) and their body mass during the post-hatch period. The research was conducted on the Eurasian blue tits, Cyanistes caeruleus, living in a nest-box area in the Niepoomice Forest (near Krakow, S Poland) during 3 years: 2009, 2010 and 2011. Rainfall did not directly affect the incubation period, but nestlings from eggs that incubated longer had a higher mass. Later hatching date was beneficial for the chicks' body mass, while earlier hatched chicks responded more efficiently to the PHA antigen. Although the intensity of rainfall did not directly affect the volume of food brought by adults to the chicks, its greater amount was beneficial for both indicators of their condition. With a more intensive rainfall, the weaker chicks responded to the PHA antigen, but achieved greater body mass. In the next years, it can be expected that heavy rainfall will become an increasingly important environmental cue, based on which birds will adapt their reproduction, and which will affect population trends, as the immune response weakened during the intensive rainfall is a survival indicator.