NEST SWITCHING AND ALLOPARENTAL CARE IN COLONIAL WHITE STORKS

White stork, Ciconia ciconia, chicks were observed to abandon their na tal nests prior to independence and to be adopted by neighbouring fost er parents in approximately 40% of broods at three breeding colonies. Nest switching coincided with a decrease in feeding rates by parents a nd an increase in aggression by siblings triggered by the flight exerc ises of nestmates, and mainly affected the eldest chicks in larger bro ods. Chicks that abandoned their natal nests moved into broods contain ing both fewer and younger chicks, and thus experienced a decrease in aggression rate compared with their natal broods. Adopted chicks may a lso have increased their food intake as a consequence of nest switchin g. Resident chicks suffered from lower feeding rates during adoption a nd had lower mass increments than chicks in non-fostering broods of co mparable age and size. Both adults and chicks were highly aggressive a gainst trespassing chicks. The probability of aggression was a good pr edictor of the extent of fostering across different brood sizes, sugge sting 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 th is species can be understood as the equilibrial outcome of an intraspe cific 'co-evolutionary' arms race between kleptoparasitic chicks and f ostering host adults with weak selection pressures for both parties.