Biparental care: short-term manipulation of partner contribution and brood size in the starling, Sturnus vulgaris

Abstract

© 1990 International Society for Behavioral Ecology In species with biparental care, the evolutionarily stable investment strategy depends, among other factors, on the reproductive value of the brood and on the contribution of one's partner. A shortfall in work rate by one parent should be partially compensated for by its partner, while both parents should invest more effort when there are more young. We simultaneously tested these predictions by attaching weights to one member of pairs of European starlings (Sturnus vulgaris), thus reducing one partner's nest visitation rate, and by daily manipulation of brood sizes (between three and seven chicks). Both sexes compensated to an equivalent degree for their partner's lower work rate, while at the same time adjusting flexibly to the daily brood size changes by increasing visit rates to larger broods. However, this ability to compensate was limited in the larger brood sizes, perhaps due to an upper limit on provisioning rate. Weighted birds and their partners showed differences in prey types delivered, relative to controls, taking a lower proportion of leatherjackets. With regard to the proximate cues affecting parental provisioning rate, begging noise levels (automatically recorded by computer from microphones in the nest-box) increased with brood size, but average length of begging bout did not. As expected, visit rates per chick decreased as brood size increased, and this was reflected in lower weight gains per day by chicks in larger broods. These results agree in general with games theory models but reveal important departures from our previous work with respect to (1) parents reaching an apparent ceiling to nest visitation rate at high brood sizes and (2) the ability to track these brood demands with short-term adjustments. [Behav Ecol 1990; 1:116-124]