Anita. Behav., 1981, 29, 878-888 HONEYEATER FORAGING: A TEST OF OPTIMAL FORAGING THEORY BY GRAHAM H. PYKE* School of Biological Sciences, University of Sydney, N.S.W. 2006, Australia Abstract. Honeyeaters (Meliphagidae) were observed foraging for nectar from Lambertia formosa inflorescences, each of which has seven flowers. The frequency distribution of numbers of flowers probed per visit to an inflorescence was found to be bimodal, with one peak at two and the other at seven. It is hypothesized that this frequency distribution results from a rule of departure from in- florescences that maximizes the net rate of energy gain. Patterns of nectar distribution were determined for a large sample of inflorescences. In addition the extent to which the honeyeaters re-probe flowers during a visit to an inflorescence was estimated. From these data and from field measurements of the times required by the honeyeaters to perform the various foraging behaviours, computer simulations of honeyeater foraging were constructed. These simulations led in turn to optimal frequency distri- butions of numbers of flowers probed per inflorescence that were bimodal but had peaks at 1 and 7 instead of 2 and 7. Although the observed and predicted behaviour were consequently similar, the difference between them was nevertheless significant. This difference could have been due to the birds' transient occupancy of the study area. During recent years many authors have attempted to use what is now known as optimal foraging theory to understand animal foraging behaviour (see Pyke et al. 1977 and Krebs 1978 for reviews). In developing this theory it is first hypothesized that animals will forage in ways that maximize their Darwinian fitness (i.e. their contribution to the next generation). In most cases it is then assumed that the appropriate 'currency' of fitness (Schoener 1971) is net rate of energy gain. This leads to the hypothesis that this net rate of energy gain is maximized. The aim of the present study was to determine whether honeyeaters, Australian nectarivorous birds (Meliphagidae), forage in accordance with this hypothesis. While foraging, animals must make many de- cisions. Honeyeaters and other nectar-feeding animals, for example, typically encounter flowers in infloreseenees and must therefore decide, after probing each flower, whether to move to another flower on the same inflorescence or to another inflorescence. This aspect of honeyeater foraging was the subject of the present study. The problem of when an animal should leave one place (patch) and move to another was first considered theoretically by Charnov (Krebs et al. 1974; Charnov 1976). In his model, Charnov assumed that an animal gathers food or energy at a continuous but declining rate during the time it spends in each patch. He showed that such an animal will maximize its net rate of energy gain while foraging if it leaves each patch *Present address: Department of Vertebrate Ecology, The Australian Museum, P.O. Box 285, Sydney, N.S.W. 2000, Australia. as soon as the instantaneous or 'marginal' net rate of energy gain in the patch has dropped to the overall rate in the habitat. In other words, if the animal can do better elsewhere it should leave. Charnov (1976) termed this result the 'marginal value theorem'. More recently, I con- sidered a somewhat different situation: not continuous and deterministic like Charnov's, but discrete and stochastic instead (Pyke 1978, 1980a). I assumed that an animal obtains vary- ing amounts of energy from discrete resource points that occur in clusters. I proposed, without proof, that this animal's optimal rule of de- parture from a cluster should have the following properties. (a) Given that the animal decides, after visiting a resource point, to move to another in the same cluster, the ratio (r) of the expected net energy gain at the next resource point to the expected time that would be required to make the move and feed there should be equal to the overall net rate of energy gain in the habi- tat. (b) Given that the animal decides to leave the cluster and move to a resource point in another duster, the ratio (r) should be less than the overall net rate of energy gain. I applied this analogue of Charnov's marginal value theorem to the foraging behaviour of hummingbirds (Pyke 1978, 1980a). In this case the resource points were fowers and the clusters were inflo- rescences. Agreement between observed and pre- dicted behaviour was good. In the analysis below I shall apply this theorem to honeyeater forag- ing. Once again the resource points will be flowers clustered on inforescences. 878