Abstract. White stork, Ciconia ciconia, chicks were observed to abandon their natal nests prior to independence and to be adopted by neighbouring foster parents in approximately 40% of broods at three breeding colonies. Nest switching coincided with a decrease in feeding rates by parents and an increase in aggression by siblings triggered by the flight exercises of nestmates, and mainly affected the eldest chicks in larger broods. Chicks that abandoned their natal nests moved into broods containing both fewer and younger chicks, and thus experienced a decrease in aggression rate compared with their natal broods. Adopted chicks may also have increased their food intake as a consequence of nest switching. Resident chicks suffered from lower feeding rates during adoption and had lower mass increments than chicks in non-fostering broods of comparable age and size. Both adults and chicks were highly aggressive against trespassing chicks. The probability of aggression was a good predictor of the extent of fostering across different brood sizes, suggesting that defence by residents was effective at preventing adoption. Residents appeared not to recognize kin but were capable of aggressive discrimination against non-familiar chicks. Fostering behaviour in this species can be understood as the equilibrial outcome of an intraspecific 'co-evolutionary' arms race between kleptoparasitic chicks and fostering host adults with weak selection pressures for both parties.