The origin and early evolution of birds

Birds evolved from and are phylogenetically recognized as members of the theropod dinosaurs; their first known member is the Late Jurassic Archaeopteryx, now represented by seven skeletons and a feather, and their closest known non-avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved from the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree-climbing characters, and its wing planform and wing loading do not resemble those of gliders. The ancestors of birds were bipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. Apart from a perching foot and some skeletal fusions, a great many characters that are usually considered ‘avian’ (e.g. the furcula, the elongated forearm, the laterally flexing wrist and apparently feathers) evolved in non-avian theropods for reasons unrelated to birds or to flight. Soon after Archaeopteryx, avian features such as the pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws with a reversed, fully descended and opposable hallux, indicate improved flying ability and arboreal habits. In the further evolution of birds, characters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are more diverse and numerous than thought previously and the most diverse known group of Cretaceous birds, the Enantiornithes, was not even recognized until 1981. The vast majority of Mesozoic bird groups have no Tertiary records: Enantiornithes, Hesperornithiformes, Ichthyornithiformes and several other lineages disappeared by the end of the Cretaceous. By that time, a few Linnean ‘Orders’ of extant birds had appeared, but none of these taxa belongs to extant ‘families’, and it is not until the Paleocene or (in most cases) the Eocene that the majority of extant bird ‘Orders’ are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden ‘bottleneck’ in diversity that fostered the early Tertiary origination of living bird ‘Orders’.     

鸟类起源于基干的初龙类

中亚三叠纪和中国早白垩世的新的重要化石发现或许可以帮助解决有关鸟类起源的争议。鸟类的飞行可能起源于三叠纪一些小型的、四脚滑翔的初龙类。羽毛的起源最初是为了滑翔而不是保温。“手盗龙类”实际上起源于鸟类 ,并至少发育了初级飞羽 ,具备滑翔的能力     

The evolution of communal roosting in birds: origin and secondary losses

Three main benefits are thought to underlie communal roostingin birds: a reduction in thermoregulation demands, a decreasein predation risk, and an increase in foraging efficiency. Iinvestigated interspecific variation in communal roosting tendenciesacross categories of several ecological factors to examine therelevance of each functional hypothesis in the evolutionary transitionto communal roosting and the secondary reversal to solitary roostinghabits. The study phylogenetic tree included 30 families and437 species. Evolutionary transitions to communal roosting occurredmore often on branches with flocking species and with largerspecies but were not associated with diet, territoriality, geographicalarea, or time of day. The association with flocking activitiessuggests that increased foraging efficiency, a factor thoughtto operate through the formation of flocks, may have been akey factor in the origin of avian communal roosting. However,several transitions to communal roosting occurred on brancheswith nonflocking species, indicating that foraging efficiencymay not be the only factor involved in the evolution of communalroosting. Secondary losses of communal roosting habits occurredon several branches, with a concomitant loss of flocking behaviorand a tendency to exhibit territorial behavior and nocturnalforaging. Secondary losses suggest that communal roosting iscostly to perform and maintain and may be lost when an asocialselection regime operates. The large number of exceptions tothe above patterns may force a reevaluation of current functional hypothesesabout communal roosting in birds.     

Polyphyletic origin of toxic Pitohui birds suggests widespread occurrence of toxicity in corvoid birds

Pitohui birds from New Guinea have been found to contain a toxin otherwise only found in neotropical poison arrow frogs. Pitohuis have been considered to be monophyletic and thus toxicity is thought to have evolved once in birds. Here, we show that Pitohuis, rather than being a tight-knit group, are polyphyletic and represent several lineages among the corvoid families of passerine birds. This finding demonstrates that the ability to be toxic is widespread among corvoid birds and suggests that additional members of this radiation, comprising more than 700 species, could prove to be toxic. It is postulated that toxic birds ingest the toxin through their insect diet and excrete it through the uropygial gland, from where it is applied to the skin and feathers. Thus, the ability to become toxic is most likely an ancestral condition but variation in diet determines the extent to which toxicity is expressed among corvoid birds. Variability in toxicity levels further suggests that the main function of the toxin is that of a deterrent against ectoparasites and bacterial infection rather than being a defence against predators as initially proposed.