The Effects of Tannins and Lipids on Digestion of Acorns by Acorn Woodpeckers

Abstract

-The effects of tannins and lipids on the metabolizable energy coefficient (MEC; i.e. energy ingested minus energy excreted, divided by energy ingested) of Acorn Woodpeckers (Melanerpes formicivorus) fed acorn meal of known composition was determined by feeding trials on captive birds. Adding tannins decreased MEC, while adding acorn lipid increased it. Quebracho (a condensed tannin) had a stronger detrimental effect on MEC than did tannic acid (a hydrolyzable tannin). There was a significant interaction (associative digestion) between tannic acid and lipid, such that the detrimental effects of tannic acid were enhanced in the presence of increased lipid. Despite the detrimental effects of added tannin, MEC of acorns, which are naturally high in tannins, was similar to the MEC of crickets, which are low in tannins. The amount of energy Acorn Woodpeckers derived from acorns (net energy value) averaged 69.0% of their maximum estimated energetic content based on composition and 60.5% of their total gross energy based on bomb calorimetry. The study indicates that stored acorns are more important to Acorn Woodpeckers than previously estimated. On average, however, acorns supply only a small proportion of the energetic needs of this species during the winter in central coastal California. Received 16 January 1990, accepted