The timing of clutch initiation in Bull-headed Shrikes (Lanius bucephalus) in relation to re-nesting

Abstrakti

The date of initiation of reproduction was studied in Bull-headed Shrikes (Lanius bucephalus) to test whether this date allows time for re-nesting and successfully laying a second clutch, and whether nest predation influences chances of re-nesting. The study was conducted during the breeding season (April-July) from 1994 to 1996 in northern Japan. Only breeders that started laying eggs in April or in early May bred twice in one season. However, not all pairs with a first nest within this period necessarily breed twice. Of the 39 pairs that bred successfully in the first breeding attempt, only four pairs (10%) initiated a second nesting attempt. The timing of laying eggs in the first nesting attempt is a necessary, but not sufficient condition for the second nesting attempt. The probability of re-nesting after nest predation decreased at 3% day-1 as the date of clutch initiation progressed. Predation during the nestling stage reduces the probability of re-nesting much more than predation during egg-laying or incubation. It is likely that the degree of investment in the first nest determines whether or not shrikes re-nest after nest failure. To ensure the chance to re-nest after failure in the first nesting attempt, Bull-headed Shrikes need to start laying as early as possible.