If the number of eggs in a clutch is reduced by predators, birds have to decide whether to continue investing time and energy in this clutch or begin a new one. The aim of this study was to assess the desertion frequency of nests with an experimentally reduced number of eggs depending on the stage of incubation, degree of clutch reduction and advancement of the breeding season in the Blackcap Sylvia atricapilla. Only five-egg clutches, most typical for Blackcaps, were investigated. At the beginning of the incubation period (one day after the last egg was laid) or at the end of incubation (10 days after the last egg), two or three eggs were removed. On the next day the clutch was monitored for desertion/incubation. Blackcaps deserted clutches significantly more often when these were reduced by three eggs at the onset of incubation (56 out of 59 clutches) compared to clutches reduced at the end of incubation (33 out of 81 clutches). As the breeding season progressed there was a significant decrease in the proportion of nests that were deserted after a three-egg clutch reduction at the end of incubation. The advancement of the breeding season did not affect the desertion frequency when eggs were removed at the beginning of incubation. Blackcaps continued incubation of all clutches reduced by two eggs (n = 20), despite reduction occurring at the beginning of incubation and early in the breeding season. These results are consistent with predictions from life-history theory in general, and the brood value hypothesis in particular.
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Vol. 108 • No. 1