Class notes
BIOL2009 LT18 Birds
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Evolution of birds from dinosaurs, unique traits of birds, evolution of flightless birds as covered in the biol2009 lecture. Extra reading with sources cited.
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Birds – Adapting to FlightClass: Aves – 10 000 species
2 living superorders
Paleognathae Neognathae
Ratites ~12 living species Gallonserae – fowls, ducks,
Tinamiformes ~47 species geese ~450 species
All flightless + in Southern Neoaves (most extant birds)
Hemisphere
Phylogeny
Adaptation to flight led to rapid radiation and
origin of modern birds during Late Cretaceous
(Cooper & Penny, 1997) – period of the fossil
record is poor (Feduccia, 1996) therefore
phylogenetic relationshops of modern avian orders
based on paleontological data remain unresolved
Traditionally, extant birds classified based on
palatal structure (Pycraft 1900), supported by
nuclear and mitochondrial sequences (Zardoya and Meyer, 2001)
Phylogenetic analyses of 20 unlinked nuclear genes suggest that ratites are
polyphyletic and also suggests multiple losses of flight (previous view that ratites
are monophyletic with tinamous as sister group – tinamous now places within ratites)
(Harshman et al., 2012)
Why evolve flight?
Birds are derived theropod dinosaurs
- Cladistic systematic emphasises monophyletic evolutionary lineages
- Biochemical discovery – protein preserved in a fossil of T.rex (collagen) yielded
peptides from a variety of extant vertebrates – closest was bird peptides (Asara et
al., 2007)
- Many hypotheses: Brief flight could help escape predation or facilitate capture of
prey
Archaeopteryx lithographica
- Discovery of this fossil genus from Upper Jurassic, was recognised as missing links
between dinosaurs and birds (Huxley, 1868)
- Appeared ~155 Mya
- Skeleton has a number of bird characteristics, but still retains theropod features
,- Asymmetrical flight feathers: symmetrical wing feathers are adequate for
balancing or gliding, leaping into the air in pursuit of insects – powered flight
requires asymmetrical flight feathers to provide thrust
In extant birds – primaries (feathers on outermost portion of wing)
Symmetrical feathers widespread among coelurosaurs, but Archaeopteryx only
proavian known to possess asymmetrical flight feathers
- Archaeopteryx not the first theropod to evolve asymmetrical flight feathers –
Xiaotingia is another fossil with similar skeletal structure but feathers not well
preserved (Xu et al., 2011)
Morphology
Most of the modifications associated
with powered flight evolved within a
short period of time (less than 10
million years) in early Cretaceous
(Sereno, 1999)
Feathers
- Feathers develop from follicles in the
skin, generally arranged in tracts
(pterylae) separated by patches of
unfeathered skin (apteria)
Some species (ratites, penguins,
mousebirds) lack pterylae and feathers
are uniformly distributed over skin
- Feather composition: 90% beta-keratin
(protein related to keratin that forms
scales of lepidosaurs), 8% water, % lipids,
proteins and pigments
- Feathers anchored in skin by short
tubular base (calamus) – remains firmly implanted within follicle until molt occurs
- Long tapered rachis extends from calamus and bears barbs – barbules branch from
barbs
- The ends of distal barbules bear hooks that insert into grooves in proximal barbules
of adjacent barb
- Hooks and grooves act like Velcro to hold adjacent barbs together, forming flexible
vane
5 types of feathers
, 1) Contour feathers – typical body feathers and flight feathers
Remiges – wing feathers, rectrices – tail feathers
Large, stiff, mostly pennaceous contour feathers modified for flight
Eg. distal portions of outer primaries abruptly tapered or notched – reduces
drag on wing when it is stretched out
2) Semiplumes – intermediate of contour and down feathers
Mostly hidden beneath contour feathers – thermal insulation and to help fill out
contour of bird’s body
3) Down feathers – entirely plumulaceous feathers, rachis is shorter than the
longest barb or entirely absent
Down feathers provide insulation for adult birds of all species
Powder down feathers disintegrate into fine particles of keratin that help
contour feathers shed water
4) Bristles – specialised feathers with stiff rachis
Occur most commonly around base of bill, around eyes as eyelashes, on head or
on toes of some birds
Bristles screen out foreign particles from nostrils and eyes of many birds
Also act as tactile sense organs and possibly as aids in aerial capture of flying
insects (eg. long bristles at edges of jaws in nightjars and flycatchers)
5) Filoplumes – fine, hairlike feathers with few short barbs or barbules at the tip
Contribute to external appearance of plumage but not exposed
Filoplumes are sensory structures that aid in operation of other factors – have
numerous free nerve endings in follicle walls – transmit information about
positions and movement of contour feathers
Sensory system plays a role in keeping the contour feathers in place and
adjusting them properly for flight, insulation, bathing, display
Streamlining and weight reduction
Contour feathers help make smooth junctions between wings and body eliminating
sources of turbulence
Weight reduction: lack urinary bladder, many species only have the left ovary,
gonads of both sexes usually small – hypertrophy during breeding season and
regress when finished
Skeletal Modifications
- Avian skeleton is not lighter in relation to total body mass of a bird than is the
skeleton of a mammal of similar size – distribution of mass
- Many bones are air-filled (pneumatic)
- Skull is especially light, leg bones are heavier than those of mammals (bird’s mass is
concentrated in its hindlimbs)
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