CLASS AVES
Two arguments as two where birds arose although all agree that they
came from the Reptiles
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some think they arose from the Crocodilians
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others believe that they arose from a group of the Theropods from the Saurischia
Diapsida => Lepidosauromorpha
=> Archosauria - Crocodilians, Pterosaurs, and Dinosaurs
Dinosaurs => Saurischians and Ornithochians
Strong evidence supports the dinosaur origin
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Good fossil evidence of Theropods with bipedal locomotion
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Oldest bird Archaeopteryx was identified as a reptile until
the feather impressions noticed; also had teeth, three fingers in wings,
probably more of a glider than flier
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Class Aves - Birds
~9,000 species world-wide, ~650 in Texas, 300 in Walker Co.
Two subclasses recognized for the Class
Archaeornithes: one order containing the extinct Archaeopteryx
Neornithes: generally referred to as the modern birds further subdivided
into two Superorders
Superorder Paleognathae - ~5 orders all lacking a keeled sternum
Superorder Neognathae - ~25 orders of carinate birds (having a keeled
sternum)
Another grouping used simply for convenience
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Passerines = perching birds - one order with ~5,500 species
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Non-passerine birds = all other ~29 orders and ~4,500 species
Birds may be grouped into categories during the year because of migration
Permanent residents (PR): here all year long
nonmigratory - the same birds are always here , i.e. cardinal, blue jay
migratory - the species as always present but the population shows a regular
turnover (some stay, some go), i.e. short distance migrators like
pine warblers
Summer residents (SR): birds that show up in the summer, breed, and leave
in the winter, i.e. chimney swift which winters in the Amazon basin, indigo
bunting
Winter residents (WR): here for winter, leave to breed in summer
regular - ones you see every year, i.e. goldfinches
erratic - here some years, gone others, i.e. evening grosbeak
Transients: populations that simply move through during migrations; blackpol
warbler a long-distance migrator that winters in N. South America, breeds
in Canada
Summer visitors (SV): shows up in the summer but doesn’t breed here, simply
going through post-breeding dispersal, i.e. roseate spoonbill
Accidentals: stragglers, wandering or displaced birds resulting from unusual
behavior/actions, i.e. clay-colord robin normally in Mexico
Basic Avian characteristics
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feathers - an epidermal derivative evolved from the reptilian scale
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forelimbs modified into wings
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edentate - no teeth
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homeotherns - maintain a constant body temperature; allows inhabitation
of various areas, habitats, wider distribution
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can fly, which is reflected in a number of skeletal modifications
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keel on the sternum
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fusion and reduction of bony elements
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Bipedal organism: stands on hind limbs because forelimbs (wings) modified
for flight - form an airfoil that allows flight; flight requires generation
of “thrust” and “lift” to counter pull of gravity and “drag”
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Skeleton: has pneumatic (hollow) bones - makes bones light (reduced weight
required for flight); reduction in numbers and fusion of elements to reduce
weight and increase strength
Development of flight feathers and muscles
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1st degree remiges - flight feathers originating on hand
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2nd degree remiges originate off ulna
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keeled sternum and flight muscles largest developed structure in vertebrates:
pectoralis major pulls wing down, supracoracoideus aid in lifting wing
(both poorly developed in ratites)
Feathers - epidermal derivatives thought to have evolved fro reptilian
scales (both primarily keratin)
all feathers collectively called plumage - dead structures that grow
and are “stuck” in dead follicles along feather tracts or pterylae,
does not grow on all areas (apterylae)
Bird skin devoid of glands except for single paired uropygial glands
at base of tail - oil gland used for preening/waterproofing
Feathers contain pigments which produce primary identifying characters
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blues produced strictly by reflection of light (tindle scattering)
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feathers wear out and must be continuously replaced by moulting
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annual moult following breeding season - prenuptual moult
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some species may undergo incomplete prenuptual moult
seasonal dimorphism - change in plumage from brighter breeding to cryptic
winter plumage
sexual dimorphism - different plumage between male and female
age dimorphism - young-of-the-year look different from the adult; may
require several years to acquire adult plumage
Bird Reproduction
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All birds are dioecious with internal fertilization by cloacal kiss that
lasts only seconds; waterfowl have a penis coming off the cloaca which
assists in fertilization
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All are oviparous - probably related to flight
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Females heterogametic (ZW) where males homogametic (ZZ)
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Gonads abdominal and change in size from small during nonreproductive periods
to large during reproductively active periods
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Reproductive patterns in north temperate birds controlled primarily by
photoperiod - male testes may increase in size by 300X
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Female ovary developes only on the left side in most - follicles enlarge
as reproductive season approaches
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Most birds have a synchronous restricted breeding season (the population
all breeds at one time of the year) usually relating to increased day length
- some species stimulated by insect hatches, rainfall, etc.
Advantages and Disadvantages
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color and song may attract predators but reproduction outweighs cost
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size of territory - the bigger the territory the more benefits until a
plateau is reached
Ossenes = songbirds
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syrynx used to produce the primary song
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territoriality
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advertisement
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three points of action against an approaching female: fight, flee, mate
Courtship = any sterotype of stimuli alone or in combination (i.e. auditory,
etc.) which serves to facilitate, synchronize, and reproductively isolate
sexes of a species
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pair bonding - bonding for reproductive purposes
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monogamy - one male with one female; 92% monogomous but not necessarily
for life, many only for breeding season
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polygamy - more than one mate for each male, 2% of all species (red-winged
blackbird)
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polyandry - more than one male mte per female. ~.5% (purple gallinule)
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promiscuity - ~6% mate without relationships
Nesting results following male/female mating
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nests may consist of little more than a depression in the ground to an
elaborate structure
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females generally lay one egg per day until the full clutch is produced
- the number normally laid by a female in a breeding period (mostly 3-4,
range 2-5)
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determinate layers - produce a certain number of eggs and stop
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indeterminate layers - continue to replace eggs if some are lost
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incubation transfers heat from the parent so that the egg can develop at
a certain temperature, most around 98°F
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incubation period in most small birds 10-14 days, longer in larger birds
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hatching accomplished by chick using either egg tooth or hatching muscle
to break out of the shell
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nestlings develop along one of two primary paths
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precocial young: independent, feathered young that can thermoregulate
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altricial young: young are totally dependent on the parent for heat and
food, may be blind and bald
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most somewhere between the two extremes
Territoriality may be developed by some species during the mating effort
- an exclusive area that is defended by a male against males of the
same species: defended by song (only by the male), displays at the boundaries,
combat when all else fails
Territory types:
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Type A: male establishes a territory, attracts a female, and everything
is defended (all-inclusive) - most song birds
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Type B: a larger territory where all occurs except feeding (uncommon) -
red-winged blackbird
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Type C: small territory used only for breeding - colonial birds, herons,
gulls, purple martins
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Type D: lek behavior where males congregate to compete for mates, establish
subterritories, wait for females to select and breed
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Nonbreeding Type E (roosting) and Type F (feeding) territories may also
be established and actively defended