Sciency Thoughts: Hummingbirds

Showing posts with label Hummingbirds. Show all posts

Sunday, 13 September 2015

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.

Saturday, 26 July 2014

Blue-bearded Helmetcrest Hummingbird classified as Critically Endangered.

Birdlife International published an assessment of the conservation status of 350 newly described Bird Species for International Union for the Conservation of Nature's Red List of Threatened Species on 24 July 2014, the first such assessment by the organization. 25% of the species described are considered to be threatened, compared to about 13% of all known bird species, though this is in part due to the discovery of cryptic species; populations of birds formerly thought to be part of more widespread species, that are now understood to be genetically distinct species, incapable of reproduction with the species of which they were thought to form a population. Such cryptic species have smaller populations and more restricted ranges than they were previously thought to have, and therefore are more likely to be threatened.

One such newly described species is the Blue-bearded Helmetcrest Hummingbird (Oxypogon cyanolaemus), formerly thought to be a population of the Bearded Helmetcrest (Oxypogon guerinii) but reclessified in the 2014 edition of the HBW and BirdLife International Illustrated Checklist of the Birds of the World. Oxypogon cyanolaemus is a medium sized (11.2-12.7 cm) Hummingbird with a prominent crest, and a 'beard' of elongated purplish-blue throat feathers.

Blue-bearded Helmetcrest Hummingbird (Oxypogon cyanolaemus). John Gerrard Keulemans (1842-1912)/Wikimedia Commons.

Oxypogon cyanolaemus is known only from the mountainous Santa Marta region of northeastern Columbia, where it is thought to inhabit high-altitude paramo shrubland, though the last reported sighting of a living specimen occurred in 1946. The paramo shrubland is under threat from extensive cattle grazing, with the practice of burning scrub to create new pasture being widespread. It is therefore considered likely that any remaining populations of the Hummingbird are very small.

The former range of the Blue-bearded Helmetcrest Hummingbird (Oxypogon cyanolaemus). International Union for the Conservation of Nature's Red List of Threatened Species.

Friday, 11 April 2014

A Pan-Apodiform Bird from the Early Eocene Green River Formation of Wyoming.

Despite their very different lifestyles the Swifts (Apodidae) and Hummingbirds (Trochilidae) are thought to be closely related groups, placed within a single combined group, the Pan-Apodiformes, along with the Swift-like Tree Swifts (Hemiprocnidae). As a group the Pan-Apodiformes are thought to be most closely related to Nightjars, Potoos, Oilbirds and Frogmouths. Several Pan-Apodiform Birds have been described from the Eocene and Oligocene of Europe, but the American fossil record of the group is somewhat poorer. Despite this the group are thought to be of North American origin; the Hummingbirds are restricted to the Americas and the closest relatives of the group are at their most diverse in North America.

In a paper published in the Proceedings of the Royal Academy of Science: Series B, Biological Sciences on 1 May 2013, a team of scientists led by Daniel Ksepka of the Department of Marine, Earth and Atmospheric Sciences at North Carolina State University, describe a Pan-Apodiform Bird from the Early Eocene Fossil Butte Member of the Green River Formation of Lincoln County, Wyoming, which has been calculated to be 51 660 000 years old using Argon isotope dating.

The new Bird is placed in the genus Eocypselus, previously used to describe a single species from the Eocene of Denmark, and given the specific name rowei, in honour of John Rowe of The Field Museum in Chicago. Eocypselus rowei is an 8 cm bird preserved as part and counterpart on a divided slab. The specimen shows exceptional preservation, with a complete skeleton and intact feathering.

Eocypselus rowei, (a) main slab and (b) counterslab. Ksepka et al. (2013).

While the skeleton is almost complete, many of the bones split when the slab was broken open, so muscle insertions are not visible. The skull is poorly preserved, but has a beak of similar proportions to those of a modern Swift, but lacking the downturned tip.

Detail of the head of Eocypselus rowei. Ksepka et al. (2013).

The feathers around the dorsal part of the head, while not well preserved and hard to differentiate, retained preserved melanosomes (colour cells). These are densely packed rod-like eumelanosomes, which are typically associated with a glossy black colouring in modern birds, but occasionally are seen in iridescent colouring.

Scanning electron microscopy image of the part of the head of Eocypselus rowei, showing abundant three-dimensionally preserved eumelanosomes. Taken at the white spot in image (a). Ksepka et al. (2013).

Swifts and Hummingbirds share similar skeletal structures in their wings, with a short stout humerus and elongate carpometacarpus and phalanges, but achieve remarkably different wing shapes and highly specialized flying strategies through different muscle insertions and plumage. Eocypselus rowei has a humerus:ulna ratio within the range of modern Swifts, but a shorter overall wing-length than any Swift. The carpometacarpus and phalanges of Eocypselus rowei are shorter than those of Hummingbirds, but the primary wing-feathers are longer. This suggests a wing-shape less specialized than either the modern Swifts or Hummingbirds, and somewhat intermediate in form.

Comparison of wing structure in Eocypselus rowei (top), an extant Swift, Hirundapus caudacutus (middle) and an extant hummingbird, Archilochus colubris (bottom), with overall body outlines at right. Skeletal elements (right side) and spread wings (left side) from the same specimen were outlined and traced, with wings mirrored to create images for extant taxa. For the fossil taxon wing length was reconstructed from the leading primary, and dotted lines indicate the uncertain breadth of the wing. For comparison, wings were scaled to the same skeletal wing length and body outlines to the same head-to-tail length (excluding the long beak of the hummingbird). Ksepka et al. (2013).

Modern Swifts have shortened, reduced legs, poorly adopted to perching, while Hummingbirds have elongate hindlimbs and are confident perchers. Eocypselus rowei also has elongate hindlimbs, and appears well adapted to perching, suggesting that the Swifts lost this ability after they diverged from the Hummingbirds.

The left and right pes (feet) of Eocypselus rowei. Pedal digits are labelled I, II, III and IV. Ksepka et al. (2013).