Dinosaurs Without Bones (48 page)

Unfortunately for both the male
Gigantoraptor
and the mammals, the bower soon collapsed under the weight of the dancing
Gigantoraptor
; this was not a surprising development, as these dinosaurs weighed about 1.5 tons, or ten times as much as an adult cassowary. This accident brought his performance to an embarrassing end, and his love interest quickly retreated, presumably to find a mate with better bower-building abilities. So not only did he not get the girl theropod, but he also ended up being a home wrecker.

Given this basic knowledge about modern bird courtship traces and what to look for, a huge advantage of interpreting trace fossils of these is that they are tied to gender; few trace fossils, other than some insect and sea-turtle nests, allow for such specificity. Thus, I have a dream that some day kinky-minded paleontologists—which is to say, virtually all paleontologists—will recognize the fossilized remains of theropod or bird courtship tracks, amplifiers, bowers, or other such wooing traces.

Dust Baths, Sun Baths, and Other Traces of Dirty Birds

Feathers require maintenance for several reasons. For one, they provide nice hiding places for skin parasites or other microbiota that might like to hang out underneath them. For another, birds have glands that secrete oil onto their feathers, an especially important accessory for birds as a form of waterproofing. This means that birds must preen and otherwise groom themselves regularly by pulling out or repairing damaged feathers, or spreading oil more equally. However, sometimes grooming is not enough, so birds seek treatments to rid themselves of too much oil in some spots, or reduce
the nasty effects of lice or other arthropods that like to latch on to them for free meals.

Two self-administered therapies used by birds to rid themselves of unwanted passengers—therapies that also leave traces—are dust baths and sun baths. Dust baths are done through the following: finding a place with plenty of exposed dry clay and silt; getting a pit started by scratching with their feet; hunkering down in this pit; and rapidly fluffing feathers while throwing wings out, up, and back to toss fine-grained sediment onto their backs. Sometimes they dip into and shake their heads in the dust, too, just to make sure all feathers are covered. These actions help to break down any oil building up on their feathers, as the dust adheres to the oil and makes it easier to shed. Moreover, fluffing uncovers skin under the feathers, which allows dust to reach those areas and choke out parasites. The trace that results from a dust bath is a shallow semicircular depression slightly wider than the wingspan of the bird; sparrows make ones that are only about 10 to 15 cm (4–6 in) across, whereas turkeys’ dust baths are more than 50 cm (20 in) wide. These hollows may also hold feather impressions or tracks, but if the substrate is too dry and collapsed on itself, these details may not be preserved.

Dust bath traces are surprisingly common traces, even in urban environments. For instance, in Atlanta, Georgia, I have seen dozens of closely spaced dust baths next to bus stops. Wherever people have worn down the grass next to those stops, sparrows took advantage of these easily available dusty patches to clean themselves by getting dirty. On the opposite end of the size spectrum from sparrows, ostriches in Africa make dust baths by sitting down, scratching out a hollow with their feet, and using both their short wings and long legs to heap dirt onto their feathers. Just to make sure they don’t miss a spot, they also roll their long necks onto the ground, leaving what must look like snake-like traces in front of their main body impressions.

Sunbathing accomplishes some of the same goals as dust baths but uses solar power. When sunbathing, some birds simply lie
down on the ground and soak up rays, whereas others sit upright on branches or other perches. What they have in common is that the birds spread their wings out to their sides. Why? When I’ve seen black vultures (
Coragyps atratus
) and turkey vultures (
Cathartes aura
) adopt this posture on chilly winter mornings, I always assumed they did this to warm up. But it may not be just the vultures that are getting warmed up but their parasites, which start moving once they reach a certain temperature. This makes it easier, then, for a bird to find and evict them.

Ornithologists have noted avian sunbathing since the 1950s, but paleontologists have seemed less aware of how this behavior might connect to non-avian dinosaurs. However, paleontologist Darren Naish, in a 2013 blog post, made such a link by reviewing sunbathing in modern birds while also encouraging people to visualize and artistically depict sunbathing theropods. Naish’s post was accompanied by photographs of various species of birds—including an owl—lying on the ground with wings spread out, basking in direct sunlight. Of these, the most striking photo from an ichnological standpoint was of a secretary bird (which, incidentally, are wicked predators) lying on its tummy and with its head up, wings out, and long tail feathers behind it. Upon seeing this photo, I promptly imagined the bird disappearing, leaving only impressions of its torso, wings, and tail feathers. I then wondered how such a resting trace might act as a model for finding similar traces made by predatory feathered theropods, whether in Jurassic or Cretaceous rocks. Trace fossils like these would not only be very nice finds indeed, but also potentially interpretable as something more than just “resting.”

Of course, many small songbirds bathe in what we regard as a conventional way by using water. In such instances, the bodies of water used for bathing might vary from puddles to ponds to lakes to oceans. Anyone who owns a birdbath or has watched songbirds around puddles knows how they partially immerse themselves, and then shake their bodies to ensure that all of their feathers get wet. Although the body of water itself will not preserve traces of this
activity, moist mud or sand on their banks and shallow bottoms might record a curious series of tracks, perhaps with splatter marks on emergent areas next to the water.

So now think of, say, a 1.5-ton
Gigantoraptor
from the Late Cretaceous, one perhaps rejected by a potential mate. In an attempt to better its health and appearance, it decides to take a dust, sun, or water bath. Now envisage the marks these respective behaviors might have produced. A
Gigantoraptor
dust bath would be a relatively shallow semicircular structure, but enormous compared to those of all modern birds, probably more than 5 m (16 ft) wide. Linking this wallow to a large theropod could be tough, but doable if it also rolled its neck and head on the ground outside of the main depression, giving more anatomical clues. In contrast, sunbathing would have left body, wing, and tail impressions, with some movement blurring their outlines but not as much as in a dust bath. A water bath would perhaps show a series of walking tracks going from less saturated to more saturated (gooey) sediments, a stopping pattern, then lots of tracks in a small area caused by shuffling of the feet; water droplets on the shoreline and on top of the tracks there would complete the picture. Hopefully this mental exercise serves as yet another example of how modern bird traces can act as predictors for what trace fossils might be out there waiting to be recognized, and the behaviors that might be divined from them.

Bird Intelligence and Tools as Traces

At one time, the phrase “bird brain” was an insult hurled at someone perceived to lack common sense. Nowadays, it should be taken as a compliment. Paralleling the extreme makeover for dinosaurs showing they were not slow reptilian-like dullards, research on bird behavior in the past thirty years or so has completely changed our view of birds as simplistic automatons obeying their genetic codes. Instead, we are increasingly seeing birds more as sophisticates with their own complicated individual and social lives, language, tools, and even culture, in which parent birds actually pass down information to their young.

Nevertheless, if you are skeptical about the term “bird intelligence” and need examples, here are a few. How about recognized gradations of bird language, in which birds can tell one another through alarm calls that a possible threat is coming from above (a hawk) or on the ground (a fox)? This was verified with chickens (
Gallus gallus
), which are often impugned as the dumbest of all birds and thus deserving of roasting pans. Not complex enough behavior for you? Okay, how about when birds inform one another that not only is a human approaching but a specific individual who harassed them several years before? Or that they then teach this to their children? This sort of learning, recall, and teaching ability has been documented in American crows. Other birds that can learn individual human faces include pigeons, magpies (
Pica pica
), and northern mockingbirds (
Mimus polyglottus
); the latter can learn to associate specific faces as threats within about thirty seconds. Still not impressed? How about superb fairy-wrens (
Malurus cyaneus
) giving their chicks a “password” (a single note) as a cue for feeding, but while the chicks are still snugly inside their eggs? Researchers who verified this found that the fairy-wrens had likely evolved this behavior as a defense against feeding the hatchlings of cuckoo birds.

Given these samples, it should not be such a stunning revelation that birds are also among the few animals that use and make tools. Nonetheless, when this was first documented starting in the 1960s, it was an eye-opener for behavioral biologists. After all, the conventional wisdom at that time was that only primates used or shaped implements from their environments to accomplish a task. Although bird tools are oftentimes quite understated as traces (how many people can tell whether a wren used a cactus spine to pry out an insect?), it is still good to know about these and add them to their bird-trace checklists.

Tool-using birds include, at a minimum, New Caledonian crows (
Corvus moneduloides
), woodpecker finches (
Cactospiza pallida
) of the Galapagos Islands, Egyptian vultures (
Neophron percnopterus
) of Africa, bristle-thighed curlews (
Numenius tahitiensis
) of a few Pacific islands, the palm cockatoo (
Probosciger aterrimus
) of Australia,
brown-headed nuthatches (
Sitta pusilla
) and burrowing owls of North America, and various herons, egrets, and seagulls. Of these birds, New Caledonian crows are the most impressive of tool users and problem solvers. These crows carefully choose or shape sticks, leaves, or feathers into practical objects, which they use to acquire food. In laboratory settings, they have even used metal hooks provided by researchers, and sometimes bent these into usable shapes. Woodpecker finches latch on to cactus spines or twigs with their beaks and then manipulate these like fine surgical tools to extract insect larvae from tight spots in trees. Like New Caledonian crows, they also may modify these items—such as shortening them—to improve their utility. Egyptian vultures and bristle-thighed curlews share the practice of grabbing a rock with their beaks and throwing these at eggs to break them open. These vultures do this to ostrich eggs, whereas the curlews crack albatross eggs; intriguingly, the vultures seem to pick rocks that are egg-shaped. Male palm cockatoos grasp sticks in their beaks, which they then drum against trees to alert females that they are in the area, echoing the mating habits of beat poets who used bongos in 1950s coffeehouses. Brown-headed nuthatches acquire short pieces of bark or sticks, which they lever against the bark on tree trunks to expose insects. Perhaps my favorite tool-using bird, though, is the burrowing owl. These actually use mammal feces as a tool, by picking up pieces of bison or cattle dung, placing these in front of their burrows, and waiting for dung beetles to arrive, which they then happily devour. Lastly, a few species of herons, egrets, and seagulls go fishing by employing feathers, berries, and even bread as lures. They either dangle these items from their beaks above a water surface or drop them onto the water to attract fish, which they then nab.

So do birds have memes—culturally acquired behaviors that modify over time—as well as genes? Although acquired knowledge and tool use is certainly shared among contemporary peers and with offspring in some birds, behavioral biologists still have not verified that this knowledge is being passed down or changed over many generations. Nonetheless, our awareness of bird culture and
tool use lends to some fun (and not so crazy) speculation about whether or not Mesozoic theropods used tools, had their own way of communicating life-saving information to one another, and whether any of these resulted in trace fossils. Of course, the likelihood of recognizing tools or related traces would be minuscule, unless paleontologists found them in the mouth or hands of a dinosaur, or they noticed an appropriately sized and shaped rock sticking out of the side of a dinosaur egg. Nevertheless, all of these insights on bird behavior make us look at dinosaurs with a little more imagination, allowing us to wonder how many of their behaviors might have been shared with birds’ Mesozoic ancestors and contemporaries, and their traces.

Thus while keeping in mind bird traces as a bridge to understanding dinosaurs of the past, as well as a few examples of how birds as modern dinosaurs affect the world today, it is time to think on a grander scale with our ichnological perspectives. In the next chapter, we will explore how some dinosaur traces probably changed Mesozoic landscapes, as well as how bird traces are currently changing our landscapes. Indeed, in some instances these changes may have even affected the evolution of modern ecosystems, implying that dinosaurs left a lasting imprint that still surrounds us and likely will persist well into our future as a species.

The Living Trace Fossil

We all live in a dinosaur trace fossil. It’s not a trace fossil in the conventional sense, like an ankylosaur trackway in Bolivia, a pro-sauropod nest in South Africa, toothmarks on a bone from Canada, or a sauropod coprolite in India. Nor is it a trace made by modern cryptic dinosaurs, which no doubt will be reported breathlessly by actors playing scientists on a made-for-TV “documentary.” It’s not something more human-made, either, such as the worldwide cultural trace of dinosaurs related to their enduring and multigenerational popularity. Instead, the trace fossil is infused in the totality of terrestrial environments, what we sense from those environments, and even some of the earth resources we use. Where we live and what we do today is all somehow related to the former existence of Mesozoic dinosaurs and the continued presence of Cenozoic dinosaurs, birds.