Configuration and geometry of sap holes drilled by the White-fronted Woodpecker (Melanerpes cactorum): effects of tree structure, sap traits and plant health

Abstract

Several species of woodpecker drill holes in living trees to feed on flows of sap. We describe sap-holes drilled by the White-fronted Woodpecker (Melanerpes cactorum) on plant species in semi-arid woodlands of northern Argentina, and examine, for the first time, attributes of the plants that may help to explain the configuration and geometry of sap-holes made by a species of woodpecker. Sap-holes vary among plant species, mostly in size and shape, and in their arrangement and location on tree branches. Moreover, patterning of sap-hole are closely similar in structurally similar species, showing foraging decisions of White-fronted Woodpeckers associated with plant structure-types at a supra-specific level. In large trees, sap-holes were small, round and arranged in rows on branches or trunks of large diameter, whereas in smaller Prosopis trees, sap-holes were rectangular and located on branches of small diameter. In other species of tree and shrub sap-holes were large and irregular, and on branches of intermediate diameter. The size of holes was positively correlated with substrate diameter for small and intermediate branches of a given group of species, but was independent of diameter in tree species with holes on the trunk. The switch between sap-consumption strategies related to attributes of trees opens the possibility that White-fronted Woodpeckers drill sap-holes trying to maximise sap-harvesting.