Regression analysis performed on published observations of oxpeckers foraging for ticks on different species of ungulates revealed a significant positive relationship between number of red-billed oxpeckers per unit body surface area and species-typical body weight. The relationship suggests that the larger ungulate species maintain a higher density of tick mass per unit surface area. The impala is the smallest ungulate that oxpeckers are reported to attend. A field survey of attendance by red-billed oxpeckers on impala and four other sympatric antelope (Thompson's gazelle, Grant's gazelle, Coke's hartebeest and topi), two of which are larger than impala, revealed that only impala were attended by oxpeckers. Oxpeckers foraged upon areas of the body that impala cannot reach with their own mouths by oral grooming. The typical behavioural response of impala to oxpecker foraging was toleration or accommodation. These observations suggest that impala harbour a greater tick mass per unit body surface area than the other antelopes compared. This could reflect the fact that impala inhabit wooded areas, reportedly higher in tick density than grassland areas inhabited by the other antelopes.