Fruits — they’re sweet, juicy, and full of vitamins and wonderful anti-oxidants (or so we were told as kids). But can you live on fruit alone? No, they are not a balanced diet, and most often are rather low in caloric value and are lacking in adequate protein to maintain good health.
Out in Reservoir Woods I stumbled upon a mixed flock of Robins and Cedar Waxwings feasting on Buckthorn berries. You might recall from an earlier post (the Worm, the Pulpit, and the Thorn plant, Novemeer 29, 2011) that Buckthorn berries are toxic to humans producing tremendous gastrointestinal distress, but are tolerated by birds. However, it’s also obvious that Buckthorn berries are not high on any bird’s list of edibles because there are still a lot of them left on the trees.
And this is where the mixed flock of robins and waxwings was dining this morning.
Cedar waxwings are such a handsome bird, with their black mask, and distinctive yellow edging on the tail and red edging on their wings, as if the appendages had been briefly dipped in colored wax. Here’s a better photo of them from Wikipedia: (http://en.wikipedia.org/wiki/Cedar_Waxwing)
The Robins were enthusiastically plucking the berries from the tree, and frequently pooping out some fruity remnants. More on this digestive characteristic later.
The berries are somewhat shriveled at this point in the winter, but maintain enough sugary constituents to be attractive to the fruit eaters.
When one robin (or waxwing) moves into a particular tree, they all do, and there was quite a flurry of activity right over my head, with no opportunity for photos because the birds were just dark blobs against the sky. (I thought I should get out of the way of potential fecal plops as well.)
I followed the birds into the woods to see where they would feed next and found them down on the ground scratching around in the litter. Were they catching insects? Nope, too cold for any insects to be out or any worms to be moving up into the litter, so maybe they were finding downed fruits? The following sequence illustrates…
hmmm…anything good to eat here?
let’s check under this pile of leaves…
ooh, look what I found…
that was tasty — now to find more.
It’s true that most frugivores do supplement their diet with animal prey, but those are scarce in the winter, so fruit makes up most of their overwintering diet. Most frugivores eat a large variety of fruits, and it turns out that these fruits are also highly variable in their nutrient constituents. Some are very high in sugar content; some are higher in lipids. But almost all fruits are notably lacking in protein or other nitrogen sources. What makes this more interesting is that some frugivores (like the waxwings) are sugar specialists, and some (like the robins) are lipid specialists, and their particular preferences provide them with the most highly absorbable diet.
In fact, Cedar Waxwings 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 other frugivores (like robins) continually lose nitrogen via excretion.
The way to maximize both your caloric and essential nutrient intake is to process food quickly. Most frugivores manage to separate fruit pulp from seeds very quickly in their intestine, and purge the gut of the indigestible seeds while retaining the pulp to extract every vital nutrient. In fact, the seeds of an ingested meal of berries might appear just 20-30 min after they are eaten. That’s why you don’t want to stand under a tree where frugivores are feeding.
This winter has been kind to the frugivores: a bountiful crop of fruits produced during the long mild fall coupled with an exceptionally mild winter that demanded fall less energy expenditure. No wonder the robins have stayed in Minnesota this year instead of migrating.