Food patch use by parent birds: To gather food for themselves or for their chicks?

Abstract

1. Despite its importance in the evolution of central-place foraging strategies, few studies have distinguished between patch use by parent birds for their own consumption vs. patch use for the purpose of provisioning young in the nest. 2. Adult Palestine sunbirds (Nectarinia osea) consume nectar and arthropods, but feed only arthropods to their nestlings. This partial separation of food types allowed us to experimentally test sex-specific trade-offs in parental foraging effort between two patches. 3. A 'variable' patch on one side of a pair's territory contained only artificial nectar for parental self-feeding (in varying sucrose concentrations between 0.25 and 0.75 mol kg-1 H2O). A 'constant' patch on the other side of the territory contained artificial nectar for parental self-feeding (sucrose concentration = 0.25 mol kg-1 H2O), plus a dish of flightless houseflies for provisioning young. 4. Experimental increases in sucrose concentration in the variable patch led the parents to: (a) consume greater amounts of sucrose solution from this patch; (b) collect more flies from the constant patch to feed their young; and (c) persistently use the 0.25 mol kg-1 H 2O feeder near the dish of flies. By titration (gradually changing the sucrose concentration in the variable patch), we demonstrate that at concentrations of 0.47 mol kg-1 H2O for males and 0.55 mol kg-1 H2O for females, the birds spent an equal amount of time foraging in both patches. Therefore, sucrose concentration in the variable patch (for self-feeding only), had to be higher for the females than for the males, in order to balance the additional value females placed upon the flies provided for chick-feeding in the constant patch. 5. These results reflect previously described sex-specific parental roles in this species, with females provisioning young mainly in the nest and males patrolling more and guarding against nest predators. Furthermore, we demonstrate for the first time the fine details of alternative adaptive trade-offs in foraging effort by parent birds when using patches for self-feeding or for delivery of food to the young.