As mentioned before with the example of Oryctodromeus and other burrows credited to small dinosaur burrows in the geologic record, burrows conferred many advantages favoring dinosaur survival, or at least gave them enough time to mate and breed. However, one big question remains about burrowing dinosaurs and birds: When did theropods start going underground? This inquiry might be answered by learning more about the forms of modern bird burrows, their environments, and possible ways these traces could be preserved in the fossil record. Given such tools, paleontologists are much more likely to find Mesozoic burrows that can then be linked to either birds or non-avian theropods.
Probing Questions: Beak Marks and Other Feeding Traces
Like all other animals, birds have to eat. But when compared to most animals, they also must eat a greater proportion of food compared to their body weights, making the idiomatic expression “eat like a bird” (applied to someone who doesn’t eat very much) quite incorrect. This energy debt is owed because of generally high metabolisms, especially for birds that fly vigorously or over long distances. Extreme examples are hummingbirds, some of which can produce thousands of wing beats per minute, or Arctic terns (Sterna paradisaea), which cover more than 40,000 km (25,000 mi) in an annual migration. Fortunately for ichnologists, one of the consequences of birds needing to eat frequently is the multitude of traces this produces. Starting with their mouths and ending with their cloacas, bird-feeding traces can be found wherever birds live.
Cough pellets, gastroliths, and feces have already been discussed in loving detail, so this section will focus on a few of their other feeding traces. Among these are beak marks, and in case researchers need an excuse to go to the beach, the best places to see these traces are along sandy shorelines. In such environments, shorebirds from small to tall stick their beaks into the sand to probe for, pry out, and procure foodstuff. Much of this sustenance consists of small clams, snails, and crustaceans, which burrow into the sand in a vain attempt to avoid predation. (As far as these invertebrates are concerned, shorebirds might as well be tyrannosaurs.)
When used in combination with tracks, beak-probe patterns can even be attributed to specific birds, such as sandpipers, plovers, or sanderlings. For instance, sandpipers do double taps—bang-bang—which leave distinctive two-holed patterns. In contrast, plovers keep their beaks on a beach surface while running along at high speed, making a more continuous and connected series of holes.
With clams, oysters, and other bivalves, more subtle traces of bird predation might be on the shells themselves. Look for chipping along shell edges, revealing where shorebirds wedged in their beaks and then opened them, an action called gaping; this exhausts an assaulted clam, which eventually can’t resist any longer and opens its valves, revealing its soft goodies inside. Further evidence of gaping might be apparent where clams are still in their burrows but all that is left are two empty valves. Sometimes these valves—still joined at their hinges—have been pulled out of their burrows and left lying on a beach surface like open books, perhaps with a valve holding a small pool of clam juice. The final piece of ichnological and forensic evidence linking a shorebird with these molluscan molestations are their tracks, which help to identify which species pried into these clams’ personal lives, then ended them.
Do these methods seem a little cruel to us? Sure, but they work great for birds. In fact, here’s something even more brutal: Some of them pick up their intended victim—either a thick-shelled clam or whelk—fly up to a height of 5 to 10 m (16–33 ft), and drop it onto concrete or hard-packed sand. Thick shell or not, few molluscans can withstand this aerial-assisted assault, and their shells will either break or at least the animals will be stunned enough by the impact that they open up. I have often seen traces of this predation along the Georgia coast, in which a seagull prowled a sandflat at low tide, extracted a clam or whelk from its burrow, held on to it with its beak, took off, and then let go. In many instances, the bird left tracks directly in front of the clam or whelk’s burrow, followed by a take-off pattern. When I look a little farther away, a bounce mark and broken shell show exactly where it impacted, and trampling around the shell pieces tells of how the gull successfully dined on its gravitationally prepared meal.
Birds may also take advantage of seashores with good rock outcrops or paved beachside parking lots, using these for opening their naturally packaged seafood. I’ve watched American crows do this in the Florida Everglades, in which they drop freshwater snails onto paved roads to break them open. In urban environments of Japan, carrion crows (Corvus corone) learned a variation of this technique, used with walnuts, in which they place these on the crosswalks of busy streets for cars to run over and break. They then wait for pauses in traffic provided by stoplights, fly down to quickly eat their meals, then take off just before the light turns green.
Beak marks made by birds living away from shorelines, especially in forests or plains, are a little different, but mostly because these are preserved in different substrates. Insect-eating birds, such as grackles or starlings, systematically insert their beaks into soil or ground vegetation to find their food. Northern flickers in particular are both persistent and insidious in their ground probing. These birds find an ant nest, insert their beaks into the main burrow shaft, and open wide (gape). This causes enough of a disturbance to encourage ants to run directly toward its mouth, which is where its long tongue picks them off, rapid-fire; for dessert, it may then drill further down to get some delectable ant larvae. Either way, this probing and gaping results in a conical hole at the top of the ant nest, and its depth corresponds roughly with the length of the bird’s bill. Seeing that ant eating has been proposed as a lifestyle for some theropods, such as the Late Cretaceous Xixianykus zhangi of China, trace fossils resembling those formed by gaping might also have been preserved in fossil ant nests.
Meanwhile, above the ground and in the trees, woodpeckers and related birds either drill into trees or peel off bark to expose bountiful supplies of wood-boring insects. Bark peeling from birds can be considerable; I have seen large dead pine trees ringed by thick piles of bark on the ground, the cumulative work of insect-eating birds and not just decay. Also, seeing that eggs are such rich sources of nutrition, beak marks also may be in eggshells of other birds. Yet predatory birds do not just limit themselves to eggs for breakfast, nor do they necessarily stick with “non-avian only” policies while hunting. Sure signs of bird-on-bird predation are neat piles of feathers below roost spots, which a raptor plucked while enjoying a once-chirping treat. These piles may also have the distinctive white fecal sprays of raptors. Of the preceding traces, though, only the drill holes in trees are likely to have fossilized, and thus far nothing like these has been interpreted from the geologic record.
Less subtle raptor feeding traces are decapitated bodies of birds, or their counterparts, severed heads. One of the most unnerving predation traces I have ever seen, belonging to a bald eagle (Haliaeetus leucocephalus), was along the shoreline of a Georgia barrier island. The trace consisted of the isolated head of a laughing gull (Leucophaeus altricilla) that was lying just above the high tide mark. All of its feathers were still there and its eyes were open, while its neck had been sheared cleanly from its torso, which was nowhere in sight. No tracks were around the head, either, which made me wonder if the eagle performed its “Red Queen” impression while airborne. Regardless, it was a vivid reminder of how dinosaurs are still killing one another, and sometimes in brutally efficient ways.
Dinosaurian Bachelor Pads and Sexy Time
In evolution, not all selection is natural: much of it is sexual. Every second of every day, mates are chosen or rejected on the basis of whether or not they possess attractive qualities to justify exchanging gametes. For males, ensuring that their sperm fertilize females’ eggs sometimes means doing more than just looking good, but also requires more effort, such as courtship. In birds, courtship may involve impressive feats of singing, or flashy displays of feathers combined with dan
cing, or showing up rival males in a competition; in other words, not so different from humans. So as a male, anything that attracts and convinces a female that your genes are the best (or at least will do for now) provides an advantage over other males and will be selected. But could such efforts actually result in traces that are later recognizable as the efforts of desperate male birds attempting to entice female birds? Furthermore, could courtship traces be used as models for detecting analogous trace fossils, whether made by non-avian dinosaurs or birds?
Some tracks could qualify as traces that might be connected with pre-mating rituals. For example, nearly everyone knows about peacocks and their exhibitions of glorious tail feathers, but perhaps fewer think about what the peacocks are doing with their feet. Peacocks, in order to show off the full extent of their tail feathers, perform a slow pirouette, often defining a tight circle. This results in many overlapping tracks in the same small, semi-circular area; whether tracks overlap from right to left or left to right could be used to figure out whether the peacock turned clockwise or counterclockwise, respectively. Male plovers also make distinctive pre-mating trackways by high stepping (also called “marking time”) and placing one foot directly in front of the other. The trackways from this behavior are downright weird when compared to their normal walking trackways, consisting of parallel sets of tracks in which each track is arranged end-to-end. If another male plover is in the area, a race might happen in which the two contenders run parallel to one another, back and forth, in front of a female. The tracks from this behavior are distinct from high stepping, as the tracks would be much farther apart from one another but still would consist of two parallel trackways.
Other than tracks, male birds of different species make many additional traces that tell us they were looking for love, perhaps in all the right places. We also already learned about male kakapos and their ground amplifiers, which they use as the avian equivalents of “Mr. Microphone” systems to advertise their availability. But how about birds that go the extra distance by making something special for that little ladybird in their life? Indeed, a few birds go the route of becoming artists and engineers, going for visual impact by constructing elaborate, shiny structures that impress females with their ingenuity.
Meet the bowerbirds of Australia and New Guinea. Sometimes nicknamed the “amorous architects,” male bowerbirds build a variety of structures—bowers—that are intended to catch the eye of a prospective female. Bowers are placed into three basic categories: mats, maypoles, and avenues. Mats are like ground canvases adorned with meticulously placed leaves, stones, shells, or even beetle wings. Maypoles are vertical structures, composed largely of sticks arranged around a small tree, but can also resemble teepees. Avenues, which consist of parallel walls made of leaves, sticks, and flowers, leading to a nuptial meeting spot, seem like runways that lead to a heart-shaped bed at their ends. The intricacy of most bowers is astounding, and may include shiny human-made objects such as items made of glass, metal, or plastic. Some bowerbirds even use perspective in their handiwork, placing smaller pieces of glass or rocks at the front of a bower and larger bits toward the rear: the only examples of such artistic renderings known outside of humans. For further enticement, in an attempt to seal the deal, bowerbirds usually throw in a dance or two.
Sadly, like stick nests, these monuments to mating have extremely low preservation potential in the fossil record. Besides, an exquisitely arranged collection of leaves, flowers, and sticks will probably not retain its original integrity after weathering, erosion, and burial for it to be interpretable as a bower; however, it might excite paleobotanists. The bowers most likely to be recognized as trace fossils are those in which their makers used rock or shell collections. After all, a large grouping of perfectly identically sized and shaped stones (and ones that clearly are not gastroliths), along with snail shells and beetle wings, should catch the attention of a paleontologist encountering these in the geologic record, especially if associated with a dinosaur skeleton, or (even better) two of the same species.
Fortunately, a lack of evidence for Mesozoic bowers did not stop the imaginations of screenwriters and computer-graphics artists for the Discovery Channel, in the TV series Dinosaur Revolution (2011), from recreating a Cretaceous scene inspired by bowerbirds. This scene starred a computer-generated image of the large feathered theropod Gigantoraptor, an oviraptorosaur from the Late Cretaceous (70 mya) of Mongolia. In it, a male with gaudily colored feathers, as well as a bright blue-and-red wattle (reminiscent of a cassowary), had cleared out a shallow circular depressionandlined it with sticks and stones. Once a more drably colored (yet comely) female Gigantoraptor approached with some interest in what he had to offer, he commenced showing off his wattle and feathers and began to dance within the bower-like structure. (Amusingly enough, the dance was scored with a flamenco-like piece of music, underlining its seductive qualities.) As an added ichnological bonus, a small-mammal burrow complex was under the bower, with its mammals increasingly disturbed by the commotion above.
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, sparro
ws 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.”
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