The pattern of the evolutionary radiation of modern birds (Neornithes) has been debated for more than 10 years. However, the early fossil record of birds from the Paleogene, in particular, the Lower Eocene, has only recently begun to be used in a phylogenetic context to address the dynamics of this major vertebrate radiation. The Cretaceous-Paleogene (K-P) extinction event dominates our understanding of early modern bird evolution, but climate change throughout the Eocene is known to have also played a major role. The Paleocene and Lower Eocene was a time of avian diversification as a result of favourable global climatic conditions. Deteriorations in climate beginning in the Middle Eocene appear to be responsible for the demise of previously widespread avian lineages like Lithornithiformes and Gastornithidae. Other groups, such as Galliformes display replacement of some lineages by others, probably related to adaptations to a drier climate. Finally, the combination of slowly deteriorating climatic conditions from the Middle Eocene onwards, appears to have slowed the evolutionary rate in Europe, as avian faunas did not differentiate markedly until the Oligocene. Taking biotic factors in tandem with the known Paleogene fossil record of Neornithes has recently begun to illuminate this evolutionary event. Well-preserved fossil taxa are required in combination with ever-improving phylogenetic hypotheses for the inter-relationships of modern birds founded on morphological characters. One key avifauna of this age, synthesised for the first time herein, is the Lower Eocene Fur Formation of Denmark. The Fur birds represent some of the best preserved (often in three dimensions and with soft tissues) known fossil records for major clades of modern birds. Clear phylogenetic assessment of these fossils will prove critical for future calibration of the neornithine evolutionary timescale. Some early diverging clades were clearly present in the Paleocene as evidenced directly by new fossil material alongside the phylogenetically constrained Lower Eocene taxa. A later Oligocene radiation of clades other than Passeriformes is not supported by available fossil data.
Carotenoids have been hypothesized to facilitate immune function and act as free-radical scavengers, thereby minimizing the frequency of mutations. Populations of animals exposed to higher levels of free radicals are thus expected to demonstrate reduced sexual coloration if use of carotenoids for free-radical scavenging is traded against use for sexual signals. The intensity of carotenoid-based sexual coloration was compared among three populations of barn swallows Hirundo rustica differing in exposure to radioactive contamination. Lymphocyte and immunoglobulin concentrations were depressed, whereas the heterophil:lymphocyte ratio, an index of stress, was enhanced in Chernobyl swallows compared to controls. Spleen size was reduced in Chernobyl compared to that of two control populations. Sexual coloration varied significantly among populations, with the size of a secondary sexual character (the length of the outermost tail feathers) being positively related to coloration in the two control populations, but not in the Chernobyl population. Thus the positive covariation between coloration and sexual signalling disappeared in the population subject to intense radioactive contamination. These findings suggest that the reliable signalling function of secondary sexual characters breaks down under extreme environmental conditions, no longer providing reliable information about the health status of males.
Several life history and ecological variables have been reported to affect the likelihood of species becoming urbanized. Recently, studies have also focused on the role of brain size in explaining ability to adapt to urban environments. In contrast, however, little is known about the effect of colonization pressure from surrounding areas, which may confound conclusions about what makes a species urban. We recorded presence/absence data for birds in 93 urban sites in Oslo (Norway) and compared these with species lists generated from 137 forest and 51 farmland sites surrounding Oslo which may represent source populations for colonization.
We found that the frequency (proportion of sites where present) of a species within the city was strongly and positively associated with its frequency in sites surrounding the city, as were both species breeding habitat and nest site location. In contrast, there were generally no significant effects of relative brain mass or migration on urban occupancy. Furthermore, analyses of previously published data showed that urban density of birds in six other European cities was also positively and significantly associated with density in areas outside cities, whereas relative brain mass showed no such relationship.
These results suggest that urban bird communities are primarily determined by how frequently species occurred in the surrounding landscapes and by features of ecology (i.e. breeding habitat and nest site location), whereas species' relative brain mass had no significant effects.
Bird fossils from Turonian (ca. 90?Ma) sediments of Axel Heiberg Island (High Canadian Arctic) are among the earliest North American records. The morphology of a large well-preserved humerus supports identification of a new volant, possibly diving, ornithurine species (Tingmiatornis arctica). The new bird fossils are part of a freshwater vertebrate fossil assemblage that documents a period of extreme climatic warmth without seasonal ice, with minimum mean annual temperatures of 14?°C. The extreme warmth allowed species expansion and establishment of an ecosystem more easily able to support large birds, especially in fresh water bodies such as those present in the Turonian High Arctic. Review of the high latitude distribution of Northern Hemisphere Mesozoic birds shows only ornithurine birds are known to have occupied these regions. We propose physiological differences in ornithurines such as growth rate may explain their latitudinal distribution especially as temperatures decline later in the Cretaceous. Distribution and physiology merit consideration as factors in their preferential survival of parts of one ornithurine lineage, Aves, through the K/Pg boundary.
Cites: Proc Natl Acad Sci U S A. 2002 Oct 29;99(22):14020-5 PMID 12388778
Cites: Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15253-7 PMID 21914849
Cites: Nature. 2005 Jan 20;433(7023):305-8 PMID 15662422
Photoperiodism and circadian rhythms have been studied intensively in birds because Aves are typical seasonal breeders and diurnal animals. Light is the most important environmental factor involved in entrainment of circadian rhythms and photoperiodism. The eyes and the extraocular photoreceptors, such as the pineal organ and hypothalamus, are reported to have an important function not only for photoreception but also for circadian organization in nonmammalian vertebrates, including birds. In this report, we review the roles of the eyes, pineal organ, and deep brain as the components of the multiphotoreceptor and multioscillator system in avian circadian organization.
The evolution of the avian brain is of crucial importance to studies of the transition from non-avian dinosaurs to modern birds, but very few avian fossils provide information on brain morphological development during the Mesozoic. An isolated specimen from the Cenomanian of Melovatka in Russia was described by Kurochkin and others as a fossilized brain, designated the holotype of Cerebavis cenomanica Kurochkin and Saveliev and tentatively referred to Enantiornithes. We have previously highlighted that this specimen is an incomplete skull, rendering the diagnostic characters invalid and Cerebavis cenomanica a nomen dubium. We provide here a revised diagnosis of Cerebavis cenomanica based on osteological characters, and a reconstruction of the endocranial morphology (= brain shape) based on µCT investigation of the braincase. Absence of temporal fenestrae indicates an ornithurine affinity for Cerebavis. The brain of this taxon was clearly closer to that of modern birds than to Archaeopteryx and does not represent a divergent evolutionary pathway as originally concluded by Kurochkin and others. No telencephalic wulst is present, suggesting that this advanced avian neurological feature was not recognizably developed 93 million years ago.