I apologize, you have asked for references from sutras.
I can't provide any right now, but I will share my limited understanding.
You used a phrase, "...the result of karma..."
Conceptually, this may be throwing you off.
What happens to a person isn't as much the result of karma, it is karma.
In other words, karma isn't meting out rewards and punishments.
Karma is the dynamic experience of the interdependence of all things.
So, there isn't really any 'retribution' to speak of.
If we assume that there is a "me" and that stuff is happening to "me"
and then try to understand how karma works this way, it won't really make any more sense
than to say it's the way things occur in a dream
where, in a dream, you imagine that it is really you, so whatever happens to you thus seems real.
So, because we imagine this 'real' me, we do things which perpetuate this.
And this perpetuity has been going on for a long time,
even before the body you have today existed.
If we begin with the premise that there is no actual "me",
but merely that what we experience as "my karma" is just the coming together of uncountable events,
resulting in, for example, the temporary existence of a human body,
it's the same with the bird who poops on your shoulder. That bird is a "coming together of events" as well.
So, the situation isn't really that you cause the bird to do that
but more that the two of you share the causes of this event taking place.
The way that karma flows from one lifetime to another is not because of any continuous "me"
that has unpaid karmic bills to pay.
Think of the whole sum of everything that you "are" as a big sack of rice being carried up a steep ramp.
--meaning uncountable component parts are heaped together.
When you die, its like the seam holding that bag breaking,
and all the rice pours out and rolls down the ramp,
where it gathers again as another heap, and scooped up into another bag.
Even though it is not the same single bag of rice it was before
it retains many of the same characteristics.
if it was short-grain brown rice at the top of the ramp in the old bag, it is going to be short grain brown rice at the bottom of the ramp in the new bag.
A mistake that people make is attaching arbitrary value judgements to the effects they experience.
For example, buddhists in some countries might say.
"because of my bad karma, I was born as a female"
I asked my teacher about this sort of thing.
What he told me that what one experiences as karma is one's mental attitude about the things that happen
and not the events themselves.
The events are empty of any intrinsic reality.
This is why it is written for example, that if you are greedy in a past life
you will be born into a life of poverty in this life.
People think that means if you were greedy before, you won't have any wealth this time.
But has nothing to do with how much wealth you have.
The point is, if you were greedy in your last life,
even if you are born into a very wealthy family in this life
you will always feel as though you never have enough!
That is because greed is the 'karmic seed' previously planted.
By contrast, many people have very little material wealth
but are quite satisfied, and very generous. They are in fact very rich.
My teacher also said that the conflict between China and Tibet
started before either China or Tibet ever existed.
That is how far back the "chain of karma" goes.
If you experience something like a dog biting you,
it must have causes. But those causes are not intrinsically positive or negative.
Suppose you are driving to the airport to catch a plane
and you get stuck in traffic and you miss your flight.
Your first thought might be "oh what bad karma!"
But then, you find out later that the jet crashed.
So now you have to change that to "oh what good karma!"
but then you find out it crashed into your house, where your family was.
So now you have to change that back to "oh what bad karma!"
But then you find out that everybody in your family had gone out to the mall just a few minutes earlier
So now you have to change that to "oh what good karma!"
...and on and on.
So, we can't actually look at the appearances of events and determine "good karma' or "bad karma".
How you caused "external" things to happen to you
is a very generalized way of expressing timeless interdependence
perpetually experienced as events happening to a continuous "me".
The more you let go of clinging to the experience of "me"
the less karma is produced.
However, you can also create circumstances unknowingly, such as accidentally killing something
Like stepping on a bug.
Even though these kinds of events do not leave a strong imprint on the mind, they will also produce effects later on
But they have more to do with things like the duration of the experience of other effects.
For example, if, because of good motivation you do good things and experience a favorable rebirth,
because of accidentally stepping on bugs, that experience will be shorter.
This has nothing to do with "right or wrong"
but merely the ordinary effect.
It's like if you mindlessly forget to put your car keys where they belong,
when you go to find them later, you will spend an hour looking for them.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.