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HUMMINGBIRD NUTRITION:
NECTAR IS SWEET BUT INSUFFICIENT

As we travel around the country each summer giving "Hummingbird Morning" programs for nature centers, education groups, and the general public, we're always impressed by the huge numbers of people deeply interested in the Ruby-throated Hummingbird. We're also pleased by the breadth of their knowledge--much of which they acquired the same way we did here at Hilton Pond Center: By simply observing hummingbirds on a day-in, day-out basis. Learning about hummers by reading books or watching TV shows isn't nearly as fulfilling as making first-hand observations, so we constantly encourage folks to take notes and digital photos and to share them with us through the Guestbook for Operation RubyThroat: The Hummingbird Project, via our "Hummingbird Hobnob" listserv, or by getting trained in the hummingbird protocols offered by Operation RubyThroat and The GLOBE Program.

We always provide time for inquiries from the audience following our public presentations, and most questions typically deal with feeding hummingbirds: When to put up or take down feeders each year, what ratio of water to sugar to use, and whether hummingbirds will die if we let our feeders run dry. With regard to the latter, we jokingly ask whether folks realize there were hummingbirds in the world before Perky-Pet started making hummingbird feeders; in other words, hummingbirds can get along just fine without us and our 4:1 mix. In virtually all habitats, these amazing little birds are probably dining on the same natural food sources they and their forebears have used for tens--perhaps hundreds--of thousands of years. Hummingbirds really don't depend on us, even if we ourselves become addicted to watching and feeding them.

All text & photos © Hilton Pond Center

Virtually everybody knows that one natural food source of hummingbirds is flower nectar. After all, why else would hummers have long thin bills and feather-edged tongues except to probe deeply into flowers and lap up sweet liquid therein? However, what comes as a surprise to many in our audiences is that hummingbirds aren't just nectarivores--they're also insectivores that dine on miniscule invertebrates. On many occasions we've watched a hummer repeatedly fly out a short distance from a perch to enter a cloud of gnats from which it systematically plucked its tiny prey. We've also seen hummers hovering within shrubs and grabbing insects and spiders from leaves and twigs, or doing the same in the eaves of the old farmhouse here at Hilton Pond Center. Some observers even report hummingbirds taking small, still-squirming insects from spider webs--which may account for silk on the bill of the young male Ruby-throated Hummingbird at right.

Since most folks have never actually witnessed a hummer gnat-catching or aphid-gleaning or spider-snatching, they sometimes ask for proof that hummingbirds eat "bugs." If we are banding hummingbirds at the time, we thwart their skepticism by touching a fingertip to the cloaca of a hummer-in-the-hand and coming up with a little drop of liquid with an even smaller black spot in the middle. The liquid is hummingbird urine, but the black matter is indigestible exoskeletal parts from a hummingbird invertebrate feast.

We've often wondered just what was in those black dots that comprise a hummingbird's feces, so this week at Hilton Pond we got out some glass microslides and cover slips and actually made a few wet mounts of hummingbird droppings. Then we travelled to nearby York Comprehensive High School to use a high-quality microscope in one of the biology labs. Mounting our digital camera on the scope's eyepiece, we were even able to get a few exposures of our hummingbird's latest meal.

We're not expert at identifying tiny insects that are barely macroscopic--much less their even tinier microscopic body parts--so we simply present you with three photos, all taken at the relatively low magnification of 40x. (We regret there's nothing in the photos to provide a sense of scale; just remember that each insect part is a subset of one of the little black dots on our fingertip in the top photo.)

In the first photomicrograph (just above), there's a relatively large mass of insect parts that stayed stuck together even as we made the slide. Easily discernible, however, are a jointed insect leg on the left side of the photo, plus a folded wing at upper right. Your guess is as good as ours on the rest of the jumbled mass. The ghostly hair-like projections at the top of the frame apparently are fungal hyphae that developed overnight on the slide--a good indicator our hummingbird didn't make complete use of organic material from insects it ingested.

The second photo (above) shows the flattened wing of some unknown insect, perhaps a midge or gnat. The transparent wing appears quite flimsy with few veins, so experienced entomologists may be able to identify the insect at least to family. There's also a long stylus-like object lying under the wing; we imagine it to be the piercing mouthpart of a monster mosquito but it may just be an insect or spider spine of some sort. (NOTE: After posting this page, we heard from Penn State entomologist Dean Morewood; he suggested the wing is from a dark-winged fungus gnat in the Sciaridae, whose larvae live in mushrooms and, occasionally, plant roots. Mike Quinn, inveretebrate biologist for Texas Parks & Wildlife, concurred with Dr. Morewood and provided a photo (above right) of Bradysia paupera, a representative sciarid gnat found in Europe.

Incidentally, hummingbirds frequently expel urine with surprising force; when we have them on their backs for banding it's not uncommon for them to spray droplets a foot or more. This is why we are careful to keep our hummers' posteriors pointed in some other direction, lest our eyeglasses collect small puddles on the lenses. (The bird below left is properly aimed and temporarily secured in a paper tube for banding.)

Hummingbird urine is usually perfectly clear but sometimes shows a yellow tinge, perhaps due to carotenoid pigments from pollen the birds ingest. In hummers that have been drinking from feeders with food coloring in the mix, urine is pink to dark red in color--possibly an indication the artificial pigment is simply passing straight through them.

Although hummingbirds are indeed nectarivores, it's unlikely they would survive for long on a diet consisting solely of carbohydrates, i.e., the sugars they get from flowers or our backyard feeders. Granted, they can convert some carbs into fats that burn efficiently and for longer periods, but they also need a nitrogen source from which they can make protein required to build muscle mass or replace feathers. Pollen they pick up while nectaring does supply protein, but probably not enough. Curiously, after sugars (including glucose, fructose, and sucrose), the most common chemicals in flower nectar happen to be amino acids, but even these nitrogen-rich plant compounds apparently aren't sufficient for a well-rounded hummingbird diet. Thus, hummers also evolved as insectivores that grab insects from the air or pluck them from flowers as they dive in for nectar. Such prey items are tremendous sources of fat, protein, nitrogen, and amino acids--the very things hummingbirds need to build strong bodies, make baby hummingbirds, and zip to far-off tropical areas to spend the winter.

Here at Hilton Pond, there are plenty of flowers, feeders, and flying insects, and examination of the nearly microscopic droppings from local Ruby-throated Hummingbirds indicates our hummers are getting all the natural nutrition they need to grow, function, and reproduce.

All text & photos © Hilton Pond Center

POSTSCRIPT: The above installment of "This Week at Hilton Pond" has turned out to be one on the most-visited photo essays on our Web site. We can only imagine why kids (and maybe adults) around the world are typing "hummingbird poop" into Google and coming straight to this page.

To post a hummingbird observation, please visit the Guestbook for
Operation RubyThroat: The Hummingbird Project

To join the "Hummingbird Hobnob" listserv, just send an E-mail

For more information about the hummingbird observation protocols for Operation RubyThroat, refer to our pages on The GLOBE Program