Fall must be like Christmas for Cedar Waxwings and Robins, who gorge themselves on the super-abundance of tree fruits available at this time of year. We found a small flock of adult and juvenile Cedar Waxwings doing just that at the MN Valley National Wildlife Refuge last week.
Learning by trial-and-error which fruits are not only nutritious, but won’t poison you, is a long process; copying what your parents feast on is much easier and safer.
Not all fruits are equal: some plants have co-evolved with fruit-eating birds and mammals that aid in disseminating the plants’ seeds; other plants evolved disseminating mechanisms that involve propelling their seeds through pressure-driven or wind-driven means and protect their valuable offspring with toxic chemicals. In addition, fruits are generally low in caloric value (compared to a nut), and may lack sufficient nitrogen for their consumers’ diets.
So, how does a Cedar Waxwing survive on a fruit diet?
Cedar Waxwings seem to be inordinately fond of high-sugar fruits, and they have exceptionally good absorptive capacity for sugars, not only in their small intestine, but in the proximal part of their colon as well (an unusual place to transport sugar). It is thought that the accelerated sugar transport capacity of their intestine actually enhances amino acid transport as well, because these birds can actually remain in positive nitrogen balance on fruit diets that cause other frugivores (like robins) to lose nitrogen via excretion.
An interesting corollary of this high fruit diet is that cowbirds (brood parasites written about in an earlier post) that lay their eggs in the nests of Cedar Waxwings have doomed their offspring, because cowbird chicks can’t survive on a fruit diet, like the waxwing chicks can.