Influence of Male and Female Quality on Clutch Size in Tits (Parus Spp.)

Abstract

Field experiments were designed to examine the effects of parental ability on clutch size, for the two sexes separately. To handicap parental ability we removed feathers from the wings and the tail of three tit species (Parus spp.) during the initial incubation period. We then removed their first clutches, and examined the effect of the handicap on the subsequent nesting. In Blue Tits (P. caeruleus), and Coal Tits (P. ater), handicapped females reduced the size of the repeat clutch more than control females. In Blue Tits handicapped females fed their young at a lower rate, lost more body mass during the nestling period, and at fledging time their broods were significantly smaller than those of controls. Their fledglings had a body mass, similar to the control fledglings. Handicapped female Great Tits (P. major) only reduced their clutch size a small amount; consequently, at fledging time, their body mass and those of their young were lower than for controls. The males of the three species of tits did not seem to increase parental investment when their mate was handicapped. When male Blue and Coal Tits were handicapped their mates did not reduce clutch size more than did those of the control group, despite a reduction in parental investment by handicapped males. However, breeding success was not significantly affected by handicapping males. We conclude that the clutch size determination of female tits is primarily based on their own ability to feed their young, and not on the quality of their mate. Male tits seem to have a lesser influence on breeding success than females, and males seem less willing or less able to increase their parental investment.