Most frequent realms for post-human rebirth
Then the Exalted One,
taking up a little dust
on the tip of his finger-nail,
said to the monks:

"Now what think ye, monks?

Which is the greater,
this little dust I have taken up
on the tip of my finger-nail,
or this mighty earth?"

"Greater, lord, is this mighty earth.

Exceeding small
is this little dust taken up
on the tip of the Exalted One's fingernail:
it cannot be reckoned,
it cannot be compared therewith.

It does not come to the merest fraction
of a part of it
when set beside the mighty earth, -
this little dust taken up
on the tip of the Exalted One's finger-nail."

"Just so, monks,
few are those beings that are reborn among men:
more numerous are these beings
that are reborn other than men.

What is the cause of that?

It is through not seeing four Ariyan truths.

What four?

The Ariyan truth of Ill,
the Ariyan truth of the arising of Ill,
the Ariyan truth of the ceasing of Ill,
the Ariyan truth of the practice that leads to the ceasing of Ill.

Wherefore, monks, an effort must be made to realize:

'This is Ill'.

'This is the arising of Ill.'

'This is the ceasing of Ill.'

This is the practice that leads to the ceasing of Ill.'"

—http://buddhadust.net/dhamma-vinaya/pts/sn/05_mv/sn05.56.061-070.wood.pts.htm

and the suttas that follow.