Hummingbird nesting success improved by the presence of Hawks.

Theoretically ‘trait-mediated trophic cascades’ occur when the behaviour of top predators affects the behaviour of smaller predators, and therefore the species upon which these predators feed. This is essentially what is seen in ‘Tom & Jerry’ cartoons, with the Mouse, Jerry, able to escape the Cat, Tom, by hiding in the kennel of the Dog, Butch. However such behaviour is notoriously hard to detect in wild populations, not least because Human intervention has removed top predators from many environments. One group that theoretically ought to be particularly prone to such effects are Birds, whose nesting success is strongly reduced by the presence of predators, and who are therefore extremely sensitive to such factors.

In a paper published in the journal Science Advances on 4 September 2015, a group of scientists led by Harold Greeney of the Yanayacu Biological Station & Center for Creative Studies, the Department of Biology at the University of Nevada, Reno and the Department of Natural Resources, University of Arizona, discuss the results of a study into the behaviour of Black-chinned Hummingbirds, Archilochus alexandri, Mexican Jays, Amphelocoma wollweberi, Northern Goshawks, Accipiter gentilis and Cooper’s Hawks, Accipiter cooperii, around the  Southwest Research Station in the Chiricahua Mountains in Cochise County in southeastern Arizona.

The study area comprised open woodland, with stands of tall trees, predominantly Juniper, Juniperus deppeana, Oak, Quercus spp., and Sycamore, Platanus wrightii, reaching between five and thirty meters in height, surrounded by open grassland. This grassland is maintained by grazing, but is dotted with saplings of the larger trees as well as other types of shrubs and bushes.

Mexican Jays are a major predator of Hummingbird nests, but are themselves prey to both forms of Hawk. The Hawks preferred method of attack is to wait on a high perch for prey then fall onto it from above in a rapidly descending pursuit. Attacks in which the Hawks chased the Jays from the same or lower heights were far less successful. Neither Hawk species showed any interest in the much smaller Hummingbirds.

A Mexican Jay removing the eggs from the nest (middle-lower right) of a Black-chinned Hummingbird. Greeney et al. (2015).

All tall trees are theoretically hazardous to the Jays, but trees with a Hawk’s nest (both species prefer to nest high in tall trees) presented an obvious source of danger. Greeney et al. found that the Jays avoided a conical zone beneath such nests, presumably to avoid predation. Hummingbird nests within these conical zones were therefore far less likely to be predated, and the Hummingbirds appeared to nest in clusters beneath the Hawk nests (80% of Hummingbird nests were found within the conical zones), though whether they responded to the presence of the Hawks or simply returned each year to sights they had found to be safe (Hawk nests are re-used for many years) was unclear.

Stylized graphical model of cone-shaped space surrounding active Hawk nests, within which Hummingbird nests had significantly higher survivorship. Data on the locations of Jays in relation to each plot’s Hawk nest were pooled across plots and were used to generate the shape of the cone, using the lowest individual jays detected during the study and superimposed on a fictional landscape representative of the study area. Yellow, Hawk nest; green, successful Hummingbird nest; red, depredated Hummingbird nest. Greeney et al. (2015).

Hummingbird nests within the cones were far more successful than nests outside the zones, fledging a significantly higher number of chicks. Four Hawk nests were abandoned during the study, and the success rate of the Hummingbird nests within the zones beneath these abandoned nests fell to almost zero, suggesting that the improved fledging rate was indeed due to the presence of the Hawks.

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Nesting behavior and parental care in an Ornithischian Dinosaur?

Protoceratops was a primitive ceratopsian dinosaur (the ceratopsian dinosaurs were quadruped dinosaurs with distinctive horns and frills at the back of their skulls), discovered in Mongolia in the 1920s by American palaeontologist Roy Chapman Andrews. They are well known to scientists as a large number of specimens have been discovered, often well preserved and in groups. This was due to the environment in which they lived, a desert with rolling sand dunes, where they had a good chance of being rapidly buried (not always after they died) and then preserved as fossils.

Protoceratops lived between about 75 and 71 million years ago in the interior of north-east Asia. Unlike more advanced members of the ceratopsid group, Protoceratops lacked large horns and a well developed neck frill. It was also much smaller than later ceratopsians, about the size of a sheep; by comparison the better known Triceratops could reach 9 m in length and stood 3 m at the shoulder, more than twice the size of a White Rhinoceros. There are currently two recognized species of Protoceratops, the original Mongolian species, P. andrewsi, named after its discoverer, and a slightly larger species from Inner Mongolia (to the south), P. hellenikorhinus, which had a slightly larger frill, better developed (though still small) horns and premaxillary teeth (which P. andrewsi lacks). Some scientists consider Protoceratops to be a single, sexually dimorphic species, with the apparent difference in distribution being due to sampling error (i.e., not enough samples have been discovered to show that the two types coexisted in the same space).

The November edition of the Journal of Paleontology (the journal of The Paleontological Society) contains a paper by a team lead by David Fastovsky of the Department of Geosciences at the University of Rhode Island, which describes the discovery of a nest of juvenile Protoceratops, apparently buried alice during a sandstorm. The nest contains 15 well preserved infants, too well developed to be hatchlings, but probably less than a year old, all apparently facing in the same direction, towards the prevailing wind.

The Protoceratops nest. The arrow gives the direction of the prevailing wind, and the scale bar is in centimeters.

Sedimentologists (geologists that study sedimentary rocks) are able to determine wind direction in ancient eolian (wind blown) sediments from the shape of the dunes. Dunes form as grains of sand are blown up the windward slope, leading to a gentle slope in the direction the wind is coming from. However this cannot climb infinitely, so on the leeward (away from the wind) side there is a sharper slope, where the grains reach the top of the dune, then collapse down the other side in a series of small avalanches (occasionally trapping small dinosaurs).

A sand-dune migrating with the wind has a distinct shape.

The three dimensional shape is hard to spot in sedimentary rocks, but the distinctive cross section is easily recognized.

Identifying infant dinosaurs is not always easy. The distinctive features of animals like Protoceratops are not often present in infant animals. As such it would be difficult to distinguish a juvenile Protoceratops from the juvenile of another primitive ceratopsian, such as Psittacosaurus, Liaoceratops or Auroroceratops. However none of these species have the same physical or temporal distribution (i.e., they were not found in the same time and place), so Fastovsky et al. have tentatively assigned the specimens to the species Protoceratops andrewsi.

Ruling out P. hellenikorhinus was even more difficult, as this species lived at the same time and (relatively) close by. The differences in size would not show in a juvenile of unknown age and development, and the horns and frills are less developed than in an adult of either species. This leaves the possibility of cutting into a skull to try to detect developing premaxillary teeth, which would be diagnostic of P. hellenikorhinus. It was deemed unwise to damage the fossils in this way, as the absence of teeth would not be evidence for the specimens being P. andrewsi, since it is unclear at what stage the teeth start to develop. It is possible that in future the specimens could be x-rayed, since the presence of teeth would determine that the infants were in fact P. hellenikorhinus, though this would be an expensive and difficult process for a relatively academic point.

The delicate skulls of the juvenile dinosaurs. Scale bar in centimeters.

Whatever the exact classification of these dinosaurs, it is clear that they were primitive ceratopsians, and that they had stayed in the nest after hatching, which has profound implications for our understanding of how these animals lived. This is because an immature dinosaur would be highly unlikely to remain in the nest unless it was receiving parental care, something that has long been a debating point among palaeontologists.

Birds, the only living dinosaurs, typically show extensive care for their young, with most species incubating their eggs with their own body-heat, and many continuing to feed the young in the nest for some time after they hatch. Crocodilians, the closest living relatives of the dinosaurs, neither incubate their eggs nor feed their young, but (unlike most other reptiles) most species attend their young at the time of hatching.

Nests containing the eggs of many species of Mesozoic dinosaur have been discovered in the past, including some with adults apparently incubating the eggs (also overrun by Mongolian sand dunes). However these incubators have always been from Therapod species closely related to birds, leaving room for speculation about the degree of parental care among dinosaurs as a whole. For some species, notably the larger sauropods, it is difficult to see how the gigantic adults could have cared for the tiny infants, though the animals are known to have travelled in packs containing individuals of differing sizes and therefore ages.

Citipati osmolskae, an Oviraptid Therapod closely related to the earliest birds, discovered apparently incubating eggs when it was overrun by a sand dune in Cretaceous Mongolia.

Ceratopsians are advanced Ornithischian dinosaurs, about as distantly related to birds as it is possible to be and still be a dinosaur. There are two possible interpretations that can be made from the discovery of parental care in Ceratopsians. Either caring for young was a primitive trait in dinosaurs (i.e. a trait found in the earliest members of the group, and persisting in their descendants - though it would still be possible for some groups to loose the trait) or it evolved separately in advanced Therapods and primitive Ceratopsians, suggesting that the trait was possible and advantageous, and therefore likely to have evolved in other groups as well. In either event this discovery is likely to fuel further research into parental care among the palaontological community.

See also Dinosaur footprints discovered in Southwest Arkansas, A new Troodontid Dinosaur with an injured toe, Dinosaur feathers preserved in amber and Dinosaurs on Sciency Thoughts YouTube.