Woodpeckers feed primarily on insects, larvae and other arthropods; however, several members of this family include plant products in their diets, such as sap. Among them, the genera Sphyrapicus and Melanerpes include the most species that specialize in sap consumption. In semiarid forests of Argentina, sap is an important food item in the diet of the White-fronted Woodpecker, Melanerpes cactorum. The aim of this study is to investigate why White-fronted Woodpeckers only consume sap from certain plants while avoiding other available plants of the same species and explore seasonality of their plant selection. We expected that combinations of plant traits (i.e. sugars concentration of sap, sap flow intensity, plant size, plant health and plant microhabitat), rather than one particular trait, determine which tree they select for sap feeding in different seasons. We examined five plant species: Sarcotoxicum salicifolium, Prosopis ruscifolia, Ziziphus mistol, Aspidosperma quebracho-bianco and Stetsonia coryne that were used most frequently for sap consumption and were consumed in all seasons by ten groups of White-fronted Woodpecker in semiarid Chaco, Argentina. Plants selected by White-fronted Woodpeckers for sap consumption were mainly larger plants that yield high sugar concentration. Of the plant species we studied, individual plant selection in all seasons was more evident in those plant species that constitute an important part of their diet (i.e. Prosopis ruscifolia and Stetsonia coryne). The selection of plants offering a greater reward in sap quality strongly suggests that the White-fronted Woodpecker maximizes food energy intake as a response to the seasonality that characterizes semiarid climates of temperate regions and conditions of food resources availability. Our results show that large trees are selected as sap trees by White-fronted Woodpecker, therefore, we recommend activities that promote retention of large trees in Chaco region.
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