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The Success of Birds is Based on Their Unique Biology

By: , Posted on: September 28, 2014

Bald EagleToday, there are about 10,000 species of birds. This is nearly twice as many species as mammals or amphibians, and more than reptiles. So many people love birds – whether they are pet owners, active bird watchers, hunters or environmentalists. According to the American Veterinary Medical Association, 3.6 million households in the USA have pet birds1.  Moreover, there are 5.8 million bird-watchers2.

Birds are not only dinosaurs – they are the only surviving dinosaurs. They are descendants of feathered reptiles3. The sequence of amino-acids in collagen from Tyrannosaurus rex demonstrates the close relationship between dinosaurs and today’s birds, and somewhat less to alligators4. Recently, it has been shown that a group of bipedal carnivorous theropod dinosaurs shrank over 50 million years5. This “sustained miniaturization” together with the skeletal innervation and feathering was essential to the development of flying birds5.

Grey HeronThere are known to have been at least 1,000 species of non-avian dinosaur. The last non-avian dinosaur lived about 66 million years ago at the end of Cretaceous period when the Earth is thought to have been hit by a large meteor. Prior to that catastrophe, there were fluctuations in dinosaur populations which may have contributed to their demise6.  The Cretaceous-Tertiary (K:T) extinction event not only saw the end of non-avian dinosaurs but also of many species. Why did birds survive?  Representatives of the existing bird orders Anseriformes, Gaviiformes, Procellariformes and Charadriformes were present in the late Cretaceous period before the extinction7.

Why are birds so successful? Like mammals, they have a four-chambered heart, homeothermy, and lungs. However, there are marked differences between the physiology of birds and those of mammals. These include the presence of nucleated erythrocytes, large yolky eggs (not found in mammals except the Monotremes), an absence of lactation (except in some taxa such doves and pigeons of the Columbiformes), and multiple patterns of parental behaviors and feathers. Birds are considered avian dinosaurs. They show marked similarities to dinosaurs such as a four-chambered heart8, laying large eggs with calciferous shells9 and calcium stores in the long bone, called medullary bones10 that is readily mobilizable for egg shell synthesis.  However, there are exceptions to this. Dinosaurs have retained the ancestral feature of having two oviducts11 compared to one in birds.

Avian Physiology I suggest that the biology of birds and their sisters, the dinosaurs, is the underlying factor allowing the success of dinosaurs over their epoch of 150 million years and of the 10,000 species of birds today. Avian physiology has multiple unique features. Birds have unique biological systems allowing flight, migration, production of large yolky eggs, maintenance of high body temperature and high blood concentrations of glucose together with care of young. To learn more on the physiology of birds, go to my revised edition of Sturkie’s Avian Physiology. This builds on a great tradition but with new chapters and new authors. I am also the new editor! I have published widely on avian biology and held senior faculty and/or administrative positions at the University of Leeds, Rutgers – the State University of New Jersey, Iowa State University, Mississippi State University and University of Wisconsin Milwaukee.



  2. accessed August 4, 2014
  3. Accessed August 4, 2014
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  5. Lee, M. S. Y.,Cau A., Naish, D. and Dyke G. J. 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345 (6196) 562-566.
  6. Brusatte, S. L., Butler, R. J., Barrett,P. M., Carrano, M. T., Evans, D. C., Lloyd, G. T., Mannion, P. D., Norell, M. A., Peppe, D. J., Upchurch, P. and Williamson,T. E. (2014). The Extinction of the Dinosaurs. Biological Reviews. In press.
  7. Macleod, N., Rawson, P. F., Forey, P. L., Banner, F. T., Boudagher-Fadel, M. K., Bown, P. R.,., Burnett, J. A., Chambers, P., Culver, S., Evans, S. E., Jeffery, C., Kaminski, M. A., Lord, A. R., Milner, A. C., Milner, A. R., Morris, N., Owen, E., Rosen, B. R., Smith, A. B., Taylor, P. D., Urquhart, E. and Yong, J. R. 1997.  The Cretaceous-Tertiary biotic transition J. Geol. Soc. 154:  265-292.
  8. Cleland, T.P., Stoskopf, M.K., Schweitzer MH. 2011. Histological, chemical, and morphological reexamination of the ‘heart’ of a small late Cretaceous Thescelosaurus.Naturwissenschaften98 (3):203-211.
  9. Buffetaut, E., Grellet-Tinner, G., Suteethorn, V., Cuny, G., Tong, H.,  Kosir, A., Cavin, L., Chitsing, S., Griffiths, P. J., Tabouelle, J. and Le Loeuff, J. (2005)Minute theropod eggs and embryo from the Lower Cretaceous of Thailand and the dinosaur-bird transition.Naturwissenschaften 00DOI 10.1007/s00114-005-0022-9
  10. Schweitzer, M.H., Wittmeyer, J.L. and Horner, J.R. 2005.  Gender-specific reproductive tissue in ratites and Tyrannosaurus rex.  Science308:1456-1460
  11. Sato, T., Cheng, Y.-N., Wu, X.-C., Zelenitsky, D. K. and Hsiao, Y.-F. 2005. A pair of shelled eggs inside a female dinosaur.  Science 308:375.

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