PROVISIONING, PARENTAL INVESTMENT AND REPRODUCTIVE SUCCESS IN JACKDAWS CORVUS-MONEDULA

In a nest box colony of Jackdaws, we measured male and female parental investment to determine: (a) its importance for the production of fledged young; and (b) its cost as a constraint on annual reproductive potential. By observing parent roles, provisioning rates, chick growth rates and annual reproductive output we found that most of the variation in annual reproductive output was caused by nestling mortality rather than clutch size or hatching success. Provisioning by the male was continuously demanding of energy and time, from incubation to fledgling, and was significantly correlated with nestling growth rate and fledgling survival. Nestling mortality was highest when the demands of provisioning on the male were greatest. Variance in nestling mortality increased between pairs in less productive years and exaggerated differences in parent quality. This evidence suggests that the ability of parents to exploit a variable food supply and provision their young sufficiently, is a key component of annual reproductive output in this species. We argue that provisioning costs might also constrain male Jackdaws to social monogamy by occupying time needed for the successful nurturing of two broods. Jackdaws maximise fledgling output through concerted investment by both parents into single annual broods. This system is comparable to that of other long-lived avian groups such as seabirds.