Bardskull

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Lucas, Alfred M. (1972). Avian Anatomy – integument. East Lansing, Michigan, USA: USDA Avian Anatomy Project, Michigan State University. pp.67, 344, 394–601. a b c Zaher, Mostafa (2012). "Anatomical, histological and histochemical adaptations of the avian alimentary canal to their food habits: I-Coturnix coturnix". Life Science Journal. 9: 253–275. Picasso, Mariana B. J.; Mosto, María C. (2018). "Wing myology of Caracaras (Aves, Falconiformes): muscular features associated with flight behavior" (PDF). Vertebrate Zoology. 68 (2): 177–190. a b c Dhouailly, D. (2009). "A New Scenario for the Evolutionary Origin of Hair, Feather, and Avian Scales". J. Anat. 214 (4): 587–606. doi: 10.1111/j.1469-7580.2008.01041.x. PMC 2736124. PMID 19422430.

LAUDER, GEORGE V. (1982-05-01). "Patterns of Evolution in the Feeding Mechanism of Actinopterygian Fishes". American Zoologist. 22 (2): 275–285. doi: 10.1093/icb/22.2.275. ISSN 1540-7063. Birds have pneumatic (air-filled) bones; the air spaces may make up the majority of the bone's volume. In mammals, bones are generally solid, or else filled with marrow. Why do birds have pneumatic bones? The traditional explanation is that pneumatic bones evolved because it makes the bones lighter, enabling birds to fly better. There are two problems with this explanation. First, many large, nonflying dinosaurs also had pneumatic bones. If pneumatic bones evolved before flight, they didn't evolve because of flight. Second, birds don't necessarily have lighter skeletons than mammals of equivalent size. Although birds' bones contain air spaces, the bone tissue is denser, so it their pneumatic bones aren't necessarily lighter than the solid bones of a mammal with the same mass. This doesn't say that pneumatic bones aren't relevant to flight, though. The hollow, dense construction of bird bones does give them greater rigidity than comparable mammal bones. However, just because pneumatic bones function well for flying, that doesn't mean that they originally evolved as a result of natural selection for improved flight. The rhamphotheca may be tough or leathery, and it grows continuously throughout the bird’s life. Structurally, this covering is a modified skin layer with an outer layer rich in keratin, the same material that makes up our fingernails. NaresTobalske, Bret W (2016). "Evolution of avian flight: muscles and constraints on performance". Philosophical Transactions of the Royal Society B: Biological Sciences. 371 (1704): 20150383. doi: 10.1098/rstb.2015.0383. PMC 4992707. PMID 27528773. A bird’s beak is one of its most defining features and one of the first things birdwatchers look at when putting a name to a face. Each bill is different, and they vary in size between barely noticeable structures like the beak of the Redpoll and the Long-tailed Tit and oversized bills like those of the Woodstork and the Toucans. Flightless Birds". SKELETONS: Museum of Osteology (Oklahoma City). Archived from the original on 2018-06-27 . Retrieved 2018-05-02.

The process of evolution by natural selection was first explained by Charles Darwin, a celebrated British Naturalist. Ironically, his study of the variation in bill shape between Finch species in the Galapagos Islands was one of the major clues for his theory! Recent Discoveries and Fossil Evidence

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Birds have extremely varied diets, which is one of the major causes of the impressive variety of bill shapes. Continue reading to learn about some important feeding strategies and the bill adaptations that make them possible. Filter feeders Smith, Nathan D. (2011). "Body mass and foraging ecology predict evolutionary patterns of skeletal pneumaticity in the diverse "waterbird" clade". Evolution. 66 (4): 1059–1078. doi: 10.1111/j.1558-5646.2011.01494.x. PMID 22486689. S2CID 42793145. Eberhard, W (2010). "Evolution of genitalia: theories, evidence, and new directions". Genetica. 138 (1): 5–18. doi: 10.1007/s10709-009-9358-y. PMID 19308664. S2CID 1409845. Changes in the hindlimbs did not affect the location of the forelimbs, which in birds remained laterally spaced, and in non-avian dinosaurs they switched to a parasagittal orientation. [36] At the same time, the forelimbs, freed from the support function, had ample opportunities for evolutionary changes. Proponents of the running hypothesis believe that flight was formed through fast running, bouncing, and then gliding. The forelimbs could be used for grasping after a jump or as "insect trapping nets", animals could wave them, helping themselves during the jump. According to the arboreal hypothesis, the ancestors of birds climbed trees with the help of their forelimbs, and from there they planned, after which they proceeded to flight. [38] Muscular system [ edit ] The supracoracoideus works using a pulley-like system to lift the wing while the pectorals provide the powerful downstroke