During development in fluctuating environments, phenotypes can be adjusted to the conditions that individuals will probably encounter later in life. As developing embryos have a limited capacity to fully capture environmental information, theory predicts that they should integrate relevant information from all reliable sources, including the social environment. In many oviparous species, embryos are able to perceive cues of predator presence in some circumstances, but whether this information is socially transmitted among clutch mates—promoting phenotypic adjustments in the whole clutch—is unknown. Here, using an experimental design for which we modified the exposure to some, but not all, embryos of the same clutch to cues of predator presence (that is, alarm calls), we show that exposed embryos of the yellow-legged gull (Larus michahellis) and their unexposed clutch mates showed similar developmental changes that were absent in embryos from control clutches. Compared with the control broods, both embryos that were exposed to alarm calls and their unexposed clutch mates showed altered prenatal and postnatal behaviours, higher levels of DNA methylation and stress hormones, and reduced growth and numbers of mitochondria (which may be indicative of the capacity for energy production of cells). These results strongly suggest that gull embryos are able to acquire relevant environmental information from their siblings. Together, our results highlight the importance of socially acquired information during the prenatal stage as a non-genetic mechanism promoting developmental plasticity.