December 26, 2019

What's in a bird?


I
t wasn’t until I took my Australian Wildlife Biology unit in 2nd year bachelors that I began to profoundly appreciate the sheer hugeness and diversity that makes up the world of birds. It is a mysterious world, a kaleidoscopic world, a complicated world – birds have captured my heart and I never want to stop delving deeper into their world! Let’s start with a simple look into the characteristics of our avian friends…


What makes a bird a bird?
There are some defining features which will let us know if we’re looking at a member of the class Aves.

·       Wings! (even the ones that don’t fly)
·       Feathers made of keratin
·       Jaw (beak) with no teeth
·       Hard-shelled eggs
·       Light skeleton made of pneumatic bones that are reduced and fused
·       Sternum (breastbone) with keel
·       Fused furcula (wishbone)
·       High metabolic rate
·       Endothermic
·       4-chambered heart


What’s so unique about birds?

Birds are basically living dinosaurs! They are neither mammalian nor reptilian but avian… birds are birds! From the class Aves, modern birds originated 65 million years ago.



Wings
This class of organisms are unique in that they have wings. These wings are equipped with keratinised feathers. Even the seemingly wingless emu and cassowary… although they’ll never be able to grace the skies, they do still have vestigial wings – wings that are remnants from their ancestors. This can be confirmed by the presence of a small claw at the tip of their rudimentary wing-structures.


Beak
Their jaws are toothless beaks made of bone at the centre and keratin on the outside.


Lightweight skeleton

Pneumatic bone
Otherwise, how would they stay up in the air? Birds have a skeleton made of pneumatic bone, which simply means that it’s ‘airy’ – it’s more hollowed with larger air pockets, serving to increase oxygen and create an incredibly lightweight structure.

Unlike our dense bones, birds’ bones also bear a honeycomb structure on the inside, further adding to the airiness. Clever.


Reduction and fusion
Bone reduction and fusion also contributes to the lightness. Instead of having multiple little bones, they’ve been reduced in numbers through the fusion of parts of the skeleton. For example, the pygostyle is the fused caudal vertebrae, the furcula is the fused left and right clavicles (collarbone), and the tibiotarsus is the fused tibia (shinbone) and tarsus (ankle).


Centralisation
There’s more! Centralisation plays a key role in making the skeleton suitable for flying. The centre of gravity is appropriately centralised to the middle of the body under the wings – you wouldn’t want a heavy tail or a boulder of a head! That’s why long necks are capable of S-shaped retractions and they have smaller, lighter distal bones (at the extremities e.g. fingers, lower vertebrae).


Keel and sternum
And we mustn’t forget about the keel-shaped sternum. On the bird’s chest is a broad, flat keel extending from the sternum (the breastbone). If you’ve handled a hen or rooster, you’ve probably noticed the curved protruding part down the middle of the chest; that’s the keel. It is where the flight muscles attach, hence why birds that are stronger flyers tend to have keels with larger surface areas whereas birds like the emu have much smaller keels.


That’s all for today! See you in my next article Xx


Sources:

Cole, L 2019, BIOL2032 Australian Wildlife Biology, lecture: Birds: Internal Anatomy and Physiology, lecture PowerPoint slides, Taronga Institute of Science and Learning, The University of Sydney

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