This stuff is so interesting! The instinctive behaviour is also
known as a fixed action pattern or FAP which is a collection of
well-defined motor patterns that are activated in response to a
stimulus, i.e. a group more eleborate reflexes . FAPs allow the
brain to concentrate on other things as they don't need require
time or energy to be thought about. The theory is they've been
selected for because they decrease choice and decision time,
which can be particularly important to survival. There are some
cool examples here
(http://www.cerebromente.org.br/n09/fastfacts/comportold_i.htm).
the bird is continues to perform parental instinctive behaviour
in the absence of a brood of chicks because the similar stimulus
of hungry open-mouthed fish is present. Another article draws
comparisons between different animals as to how much of their
behaviour is fixed/pre-determined . There can be a correlation
between the size of the cerebral cortex, used for thinking and
problem solving, and the amount of innate behaviour (