Multiple breeding in the Great Tit, II. The costs of rearing a second clutch

Abstract

1. Multiple breeding (raising more than one batch of young per breeding season) is a common life-history tactic, but little is known as yet of the accompanying costs and benefits. Second clutches of Great Tits, a facultative multiple breeder, were removed over three years to investigate the costs of rearing a second clutch. 2. Female parents, but not males, survived significantly better than unmanipulated control birds when the second clutch was removed. The difference between the sexes indicates a potential sexual conflict over the decision to start a second clutch. In neither sex were effects found on egg production in the next year. However, birds of both sexes produced more fledglings from first clutches in the following year when the second clutch had been removed. 3. In males, the experimental effect on fledgling production in the next year could be attributed to their mates, because this effect was restricted to males that bred with the same female as in the previous year. In females, this effect was also found among birds that bred with a new mate, which suggests that rearing a second clutch had a long-term effect. Females, but not males, found roosting in the following winter had lower mass when the second clutch had been removed, which supports the conclusion that rearing a second clutch has long-term effects. 4. The experimental effect on female survival was found in two winters with low food availability, but not in a winter with high food availability. This is in agreement with the results of a non-experimental analysis of data collected in the same population. The effect on fledgling production in the next year was also restricted to years with low food availability in the intervening winter. This suggests that the costs of rearing a second clutch depend on food availability in winter. The possibility that the costs of reproduction generally depend on environmental quality and possible mechanisms are discussed. 5. Food availability in winter is probably unpredictable at the time second clutches are started, and therefore Great Tits should 'bet-hedge' with regard to the decision to start a second clutch.