Living off the Wax of the Land: Bayberries and Yellow-Rumped Warblers

Abstract

Yellow-rumped Warblers (Dendroica coronata) and Tree Swallows (Tachycineta bicolor) are among a small group of birds in temperate North America that regularly eat waxy fruits. During the autumn, winter, and spring, these species feed extensively on fruits of the bayberry (Myrica spp.). Covering the pulp of these fruits is a solid, waxy material consisting primarily of saturated long-chain fatty acids. For most animals, saturated fatty acids are poorly assimilated (< 50%). Using H-3-glycerol triether as a nonabsorbable fat marker, we determined that Yellow-rumped Warblers are capable of high assimilation efficiences (> 80%) of bayberry wax when fed berries recoated with radioactive wax tracers. Efficient fatty-acid assimilation extends to berries coated with cetyl palmitate, a common marine, saturated wax ester (> 90%). The fatty-alcohol moiety of the marine wax was assimilated with a much lower efficiency (< 50%). A beeswax coating of the berries is assimilated with an efficiency of approximately 50%. Similar assimilation efficiencies of each wax are recorded for Tree Swallows feeding on recoated bayberries. Yellow Warblers (D. petechia) rejected recoated bayberries and exhibited little (< 5%) lipid assimilation of radiolabeled lipids. Yellow-rumped Warblers possess several gastrointestinal traits that permit efficient saturated-fat assimilation. Among these are an apparent retrograde reflux of intestinal contents to the gizzard, elevated gall-bladder and intestinal bile-salt concentration, and a slow gastrointestinal transit of dietary lipids. These gastrointestinal traits permit efficient assimilation of saturated fatty acids on bayberry fruits and may allow these small passerines to maintain more northerly wintering ranges than closely related species.