When a flock of birds starts occupying a tree,
they employ a tactic of Uniform Distribution,
spreading evenly, each maintaining the most territory
at any given moment.
When people enter a bus...
The birds are not unfriendly either.
They have just travelled half way through the planet
together to arrive at this particular tree.
From gradations of psychological paths,
it is likely that Us and Them will behave
in a manner in accordance with a psychological mechanism
much more primitive that has been developed earlier in Evolution
for the purpose of maintaining survival in crowded environments.
Unless, that is,
Evolution has found reason to change this habit in later productions.
The fact that birds and humans
behave the same in this manner,
gives reason to believe,
much like with any other organ or structure,
that this particular behavior originated from an
ancestor with this mental capability,
sufficiently early in
Evolution history,
who is a common ancestor of both man and bird.
This would be at least a reptile.
It should not be surprsing to see such behaviour in much earlier forms, and might not be incidental that
such behaviour is also that of chemical particles
in gases and liquids.
It is worth noting however, that Bear Necessities,
such as flight organs, or climate oriented insulation,
have been separately developed in many species,
and only dedicated scrutiny
of the exact mechanisms and other common grounds
can tell us whether, for example,
wings of the variuos species truly have a common ancestor.
We know too little about our own
psychology,
let alone that of birds,
to draw solid conclusions in this
respect.
