I believe that birds lay eggs rather than having an internal pregnancy,
because
1. they are descandants of dinosaurs which laid eggs and
2. like so much else in birds, it is a necessity enforced by flight.
(Modern flightless birds are descendants of birds which flew). Birds
have given up many things, including jawbones and teeth, in order to
reduce the flight-load. 'External' development of young means that the
brooding parent does not have to move about carrying a heavy internal
load as mammalian parents have to (and as a mother and grandmother I
can assure any doubters that this can seriously impede brisk movement).
(I dont imply that they did this on purpose; just shorthand for saying
that this is the result of evolutionary and natural selection processes).
Many modern reptiles, such as the Bluetongue Lizard, and many snakes,
give birth to live young - technically they are ovoviviparous because
they retain the eggs inside and the young for some time after they
'hatch, while the young are still being nourished by the yolk-sac
attached to the stomach.
With eggs, though they require parental care, they are produced only one
at a time until a full clutch is achieved and incubation commences; this
is less of a strain and a burden on the gravid female than a whole
litter at once as in mammals such as pigs or dogs. If the clutch is
discovered by a predator, or overtaken by accident, very often the
brooding bird can escape and produce another set of eggs. The size of
the bird's clutch can be adapted to nutrients available. Raptors of
course start incubation with the first egg - leading to a succession of
ages and sizes of young through the clutch. If it's a good season, most
will survive. If not, the 'surplus' younger ones are likely to be eaten
or just out-competed by their elder siblings.
And if anyone wants to ask about flightless birds, it's no accident that
so many have arisen from flyong ancestors on oceanic islands, where in
addition to not needing flight in a predator-free environment,
possession of flight is downright disadvantageous - the flying bird is
much more likely than the flightless to be blown off the island and
perish during storms and cyclones.
Anthea Fleming
in Ivanhoe
wishing I could evolve a fur-coat on this chilly day.
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