Oxpecker host selection appears to be governed by an array of factors affecting the efficiency of foraging for ticks, with optimally foraging oxpeckers choosing those hosts that maximize tick intake and/or minimize search time. We studied yellow-billed oxpeckers Buphagus africanus (Linnaeus) at Matobo National Park, Zimbabwe, in order to examine the relationship between host selection and seasonal tick abundance, host characteristics and water availability. Preference ranks were highly correlated between the dry and wet seasons, implying that relative tick burdens of host species did not change appreciably. The selection index (a measure of oxpeckers per host) and the density index (a measure of oxpecker density on host body surface) showed a threefold increase from dry season to wet season for all host species, presumably due to greater tick burdens on hosts during the wet season. Host selection indices were positively correlated with species-typical host body mass, indicating that oxpeckers selected larger-sized hosts that supported higher densities of ticks. A negative correlation between host preference and herd size during the wet season suggested that oxpeckers optimize foraging efficiency by choosing larger herds when attending smaller-bodied hosts. Hosts observed at a water source appeared to be more attractive to oxpeckers than those surveyed where water was not available.