Herbivores that exploit sap have adaptations to overcome the defensive mechanisms of plants. Sap-eating animals face thick cuticles and clogging mechanisms that protect sap. Yellow-bellied sapsuckers (Sphyrapicus varius) exploit both the phloem and xylem sap of trees and are expected to have adaptations associated with this feeding habit. Field observations from breeding seasons in 1991, 1992, and 1993 at the UMBS near Pellston include 41 pairs of sapsuckers and their offspring. Hypotheses are tested about the mechanism of sap extraction and the consequences of sap feeding for reproductive success and drumming behavior. Sapsuckers harvest xylem sap from trees for a brief period in early spring. Trees with higher concentrations of sucrose but not with higher sap flow rates are preferred. Sapsuckers extract phloem sap during most of the breeding season by making large square holes in the boles of trees. Phloem sap flows from these incisions, which is surprising because incisions in sap tissue usually clog, preventing sap flow. Comparisons between trees used for sap extraction and those not used reveals that sapsuckers concentrate their efforts on individual trees. Observations show that sapsuckers repeatedly wound the tree above old wounds to form a cluster of sap holes, and may use saliva to unclog blocked phloem tissue. Experimental field manipulations demonstrate that sap feeding does not appear to influence reproducitve success as much as the availability of nesting cavities. Sapsuckers use drumming as a form of acoutic communication, but they do not advertise the quality or quantity of sap trees with their drumming. Rather, field observations and manipulations reveal that they choose substrates for drumming that can give loud sounds of a low frequency when tapped. Fewer specializations appear to be necessary to induce the flow of xylem sap than phloem sap from incisions through bark. Therefore, it is suggested that the exploitation of xylem sap may have been a precursor for the evolution of phloem-sap extraction. Many animals steal sap from extracters rather than extract it themselves and research on these animals may elucidate the evolutionary pathways leading to specialization on sap.