SUGAR PREFERENCES OF SOME SOUTHERN AFRICAN NECTARIVOROUS BIRDS

Three southern African nectarivorous passerine birds, Gurney's Sugarbi rd Promerops gurneyi, the Malachite Sunbird Nectarinia famosa and the Black Sunbird Nectarinia amethystina, were tested to determine their h exose and sucrose preferences. All three species preferred sucrose whe n offered a choice of 0.25 M solutions of glucose, fructose and sucros e. However, when the concentrations were increased to 0.73 M, the thre e species showed no preference for any of the three sugars, The choice made at low concentrations (equivalent to the lower limit of the rang e of nectar concentrations of preferred nectar-producing plants) may r eflect preference for the sugar with the highest energy reward. We als o examined the proposition that birds offered a choice of different co ncentrations of one sugar would show ranked preferences and maximize t heir rate of energy return by selectivity, In contrast to expectations , Gurney's Sugarbird and the Malachite Sunbird showed no preference fo r the highest concentrations, We suggest that dietary choices in these species indicate the birds had either reached a limit where they had sufficient energy intake or were affected by post-ingestion constraint s.