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Abstract

We conducted an experiment to test whether great tits (Parus major) base their decisions of clutch defence on past investment or future benefits. Results were obtained by manipulation of future benefits. Great tit pairs of an experimental group with reduced clutch-size and thus diminished benefits defended their offspring against a live raptor significantly less than a comparable, non-manipulated control group with the same amount of past investment (clutch-size, incubation stage, time of year). While in the females the difference between the two groups was obvious from the beginning of a trial, the difference in the males developed only during the course of a trial, suggesting that the male bases its response on the female's response deficit. A more onerous explanation of the female signaling the male the egg loss by using a language-like symbol is not supported by the data. The experiment thereby permits the conclusion that at least the great tit female avoids committing the Concorde fallacy in the strict sense.