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BLACK VULTURE DEMISE

On 6 February--while getting things ready for an end-of week departure and our annual Costa Rican hummingbird expedition--we took a short lunch break at Hilton Pond Center. As we munched our noontime meal, we left the kitchen momentarily and entered the adjoining office to fetch binoculars. Glancing out the big picture window above our computer desk, we noticed something very large and dark thrashing around on the surface of the pond. At first we thought it might be Canada Geese mating, so we immediately grabbed our Canon digital SLR with 400mm telephoto lens, opened a sliding door slightly, and started firing away. Through the camera viewfinder we could tell it wasn't geese after all but a Black Vulture moving frenetically on the surface with its four-and-a-half-foot wingspan fully extended. How odd to see this avian scavenger on a pond where water was at least a fathom deep.

All text & photos © Hilton Pond Center

At first we thought this Black Vulture might have soared too close to the pond's surface and tumbled in (above), and that the bird was trying to swim toward shore--throwing up a shower of water droplets as it flapped. However, the vulture was only bobbing up and down, making no forward progress. As we studied the scene for further clues, it seemed the bird was actually on top of some sort of carcass.

Maybe a Muskrat or one of those behemoth 40-pound Grass Carp had died and floated up, after which the vulture had landed on the deceased object and begun to peck away at its meal (above). This supposition also proved to be incorrect, as we learned when the unmistakable naked head of another Black Vulture suddenly popped from the water (below).

Through our long lens it looked like the top vulture was biting the crown of the submerged one, so now we were really confused. Nonetheless, we quickly came up with several possible explanations for the scenario we were photographing.

• The bottom bird had died earlier--perhaps while drinking from the pond bank--and floated across the surface, where it attracted the attention of a cannibalistic conspecific.
• The two vultures were territorial males chasing each other across the sky, when one bird dived too steeply and hit the pond. There it got bogged down and became an easy target for its more aerobatic aggressor.
• We were looking at a similar non-aggressive situation in which an amorous male had been in pursuit of a female that skimmed too close to the water's surface.

If the Black Vulture on the bottom was a male competitor, was the top bird finishing off his rival as he floated helplessly across the pond? Or, if the submerged bird was a female, was the top vulture attempting to mate, biting her head as some male birds are prone to do in the throes of copulation? And if the bottom Black Vulture was already dead, was its head bobbing up only because the top bird was nipping at it? Without being too anthropomorphic, is it reasonable to think the uppermost Black Vulture was somehow trying to rescue a mate--possibly one that was party to a long-term pair bond of which long-lived Black Vultures could be capable?

We couldn't be sure on any of these hypotheses, so the vultures-in-the-pond scenario will teach us not to stop for lunch; had we not been eating and instead been at our desk to witness the beginning of this mysterious encounter, we might have a better understanding of what we were watching.

In any case, while the ruckus on the pond continued we fired off photos for a minute or two. About the time when we no longer saw the lower vulture's head popping up, the top vulture became aware of our presence and--adriotly as a Wood Duck--lifted from the water's surface with graceful ease (below), apparently leaving for dead the second Black Vulture.

Since we still had no convincing explanation for what we had just observed, we walked down to the pond for a better view. From ten yards away, it was obvious that--regardless of cause--the remaining vulture was floating head down with wings extended and was obviously dead (below), but the only way to collect additional data would be to examine the carcass itself. The wind was blowing briskly, so rather than get a raft out of winter storage to collect the dead bird, we decided to wait until it simply washed up on the shore of Hilton Pond.

Because the winds were erratic, we ended up watching from our office as the vulture drifted around for a few hours before finally coming to rest in weeds near the north bank. There we fished the floater out with a long pole and grabbed its legs to lift it from the water. Each leg (below) was short, stout, and scaly, with blunt claws very different from the sharp, decurved talons of hawks and eagles. Although vultures were once classified as raptors, their feet are adapted for walking--not tearing flesh--and they're now thought to be more closely related to storks. We were surprised to see a considerable amount of red clay caked on the toes and claws. Black Vultures spend a lot of time on the ground feeding on carcasses, so this individual may have been into some soft mud on days before it perished.

Also quite noticeable on the dead bird were bright white shafts and gray vanes characteristic of a Black Vulture's outer primary feathers (below); in flight, these create the white wing tips typical of a Black Vulture, Coragyps atratus, and differentiate the species from Turkey Vultures, Cathartes aura, that have wings with black lead edges and trailing edges of gray.

The dead Black Vulture's leg morphology and wing tip color were of interest, but the first clue to what might have happened to the corpse was its eye--crystal clear and still spherical. This led us to believe the bird was very recently dead because eyes in dead birds tend to lose their shape and become cloudy rather quickly. Another possible clue was a couple of small tears in the rough gray-black skin on the dead vulture's crown. It seems likely these were caused by pecks from the vulture that escaped--just the sort of wounds that might have been expected from an encounter between two rivals, but that also could have been caused by vigorous courtship.

For our last potential clue, we had to dig a little deeper--and we mean that literally. We decided it might be useful to know the sex of the dead vulture, surmising that if it was a female we might conclude the catastrophe on the pond was the result of a courtship flight gone awry. Since Black Vultures are sexually monomorphic--i.e., males and females look alike externally--the only way to determine the sex of the carcass would be to look inside. So we got out our trusty old dissection kit, selected a large blade for our scalpel, moved aside feathers on the bird's flank, and prepared to make an incision in the intercostal space--between the last two ribs.

We chose that spot because of our experience conducting laparotomies on nearly a thousand live Blues Jays in Minnesota during grad school days. Since Blue Jays of both sexes also look similar, the only way we could definitely know the sex of each bird in our study population was to make a tiny incision through the abdominal muscle, thus allowing us to look inside the body cavity for an ovary or a testis. (We didn't need to laparotomize incubating females because they developed brood patches on their bellies.) Jays survived this carefully performed procedure with no difficulty. We always made the incision on the jay's left side because in virtually all birds the females' right ovary is vestigial--likely a weight-reducing adaptation for flight.

In any case, we didn't need to use a light-touch laparotomy technique on the already dead vulture. The procedure actually turned out to be a bit harder than on Blue Jays because the vulture's body was covered by a dense layer of down, and because muscles surrounding its ribs were surprisingly thick. In fact, we ended up removing the ribs themselves to get a clear view. As soon as they were out of the way, a stretched left oviduct practically spilled from the body cavity, revealing a one-inch-diameter developing ovum that resembled a badly bloodshot eye (below). All this was sure sign the dead vulture was not only a female but that she was reproductively active and ready to lay at least her first egg of the current breeding season.

The results of our dissection were not a surprise; vultures nest in early February here in the Carolina Piedmont and it's not unexpected to see them courting at this time of year. What IS surprising is that one of these courtship flights apparently ended abruptly when--we speculate--an aggressive male drove his prospective mate to her untimely death in the cold mid-winter waters of Hilton Pond. We can't be positive that's what happened, of course, but evidence is very strong that what we witnessed was a male Black Vulture trying to copulate on the water with a female he may have killed in the process.

All text & photos © Hilton Pond Center

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Be sure to scroll down for an account of all birds banded or recaptured during the week, plus other nature notes of interest.

"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

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