The cyanogenic glycoside amygdalin does not deter consumption of ripe fruit by Cedar Waxwings

Cyanogenic glycosides are common secondary compounds in ripe fruits that ar e dispersed by birds. These substances are toxic to some mammals. We examin ed the repellent effect of amygdalin, a cyanogenic glycoside, on Cedar Waxw ings (Bombycilla cedrorum). Amygdalin did not reduce food ingestion in Ceda r Waxwings, even at relatively high concentrations. In addition, these bird s did not exhibit preference for amygdalin-free over amygdalin-containing f ruit. Cedar Waxwings given artificial food that contained four times the am ount of amygdalin found in some wild fruits ingested the equivalent of 5.5 times the oral lethal dose for rats in 4 h without exhibiting any external signs of toxicity. Amygdalin ingestion appeared to have a negative effect o n nitrogen retention and food assimilation. However, when nitrogen retentio n and food assimilation were recalculated assuming that ail amygdalin inges ted was excreted intact, these negative effects disappeared. The presence o f large amounts of unhydrolyzed amygdalin in the excreta of waxwings fed on amygdalin-laced food confirmed our conjecture that amygdalin was excreted intact. We hypothesize that in Cedar Waxwings, amygdalin is absorbed in the intestine but is not hydrolyzed by endogenous enzymes and thus is excreted intact in urine. The apparent lack of repellent effects of amygdalin in Ce dar Waxwings suggest that toxicity data for rats and humans may be a poor p redictor for the deterrent effect of fruit secondary compounds on frugivoro us birds. Many hypotheses that have been posed to explain the presence of s econdary compounds in ripe fruit assume that these substances have repellen t/toxic effects on avian seed dispersers. For some compounds, such as amygd alin and other cyanogenic glycosides, this assumption may not be valid.