“A sutra states that both the crow’s blackness and the heron’s whiteness are actually the deep stains of their past karma.” — Nichiren, Letter from Sado
Taken literally, this statement reflects a common misunderstanding of Karma, as taught by the Buddha. 
Karma {from the verbal root kri, kha = to put, make, do} does refer to a process of causality. However, Karma is only one kind of cause and effect. The Buddha was rather clear that karma is rooted in cetana {ceta = mental state + na = the gerundive suffix -ing; motive, intention}. A cetana is the basic unit of volitional motivation. So, karma refers only to causality involving volition; willful or intentional actions.
While the quoted sentence might be construed figuratively, we know the coloration of birds really has nothing to do with volitional deeds they committed in their past lives. The Pali commentaries inferred five processes of causality from the Suttas. The Buddha did not explicitly use these terms. However, the concepts are implicit in the Suttas. Moreover, they are pretty much consistent with modern science.
The process are called niyamas. The prefix ni- is similar to ex- and generally means something like ‘out of.’ In some cases, it can mean ‘into; or can be an intensifier. Yama means something like control, regulate, or restraint. In Patanjali Yoga, yama and niyama refer to rules of conduct. In our present context, niyama indicates the processes that regulate causation. The Panca Niyama are five process of causality are:
1. Utu Niyama or Inorganic Processes: Utu simply means non-living or inorganic. In ancient India, this was about the interaction of four basic physical or elements; earth or solids, water or liquid, wind or gases, and fire or energy. Today, we know these are not really elements, but the analysis still has some merit. In modern terms, Utu Niyama refers to processes studied in the fields of geology, inorganic chemistry, climatology, and so on.
2. Bija Niyama or Genetic Heredity: Bija means seminal, germ, or seed. In modern terms, bija niyama refers to the fields of biology, organic chemistry, genetics, and so on. The crow’s blackness and the heron’s whiteness are actually the result of bija niyama; their genetic inheritance.
3. Karma Niyama or Volitional Moral Causality: I already mentioned that karma is rooted in cetana, or basic motive. Buddhism divides cetanas into the categories of kushala or wholesome, akushala or unwholesome, and neutral. That involves morality. My understanding is that cetanas become habitual or patterned. One’s habitual pattern of motives is called cetana samskara or volitional conditioning, and constitutes the fourth skandha. Karma is ‘created’ by three kinds of conduct, actions, or causes; volitional conceptual thought {manas}, volitional speech, and volitional bodily deeds. The Buddha taught the every such volitional is a cause {hetu} and invariably produces an effect or fruit {vipaka}.
Then Ven. Angulimala, early in the morning, having put on his robes and carrying his outer robe & bowl, went into Savatthi for alms. Now at that time a clod thrown by one person hit Ven. Angulimala on the body, a stone thrown by another person hit him on the body, and a potsherd thrown by still another person hit him on the body. So Ven. Angulimala — his head broken open and dripping with blood, his bowl broken, and his outer robe ripped to shreds — went to the Blessed One. The Blessed One saw him coming from afar and on seeing him said to him: “Bear with it, brahman! Bear with it! The fruit of the kamma that would have burned you in hell for many years, many hundreds of years, many thousands of years, you are now experiencing in the here-&-now!”
– The Buddha, the Angulimala Sutta
According to the seed that’s sown,
So is the fruit you reap therefrom,
Doer of good will gather good,
Doer of evil, evil reaps,
Down is the seed and thou shalt taste the fruit thereof.
– The Buddha, the Nibbedhika Sutta
4. Dhamma / Dharma Niyama or Laws of Phenomena: Dharma can mean phenomena in general; usually transliterated as dharmas with a little ‘d,’ or the Buddha Dharma; which is usually capitalized. I have seen dharma niyama taken both ways. A few take it to mean the power of Dharma. Most, including myself, take this to mean the former, corresponding to the Laws of Physics; such as Gravity, Conservation of Mass-Energy, Conservation of Momentum, Laws of Thermodynamics, and so on.
5. Citta Niyama or Power of Mind: Note that Citta is an inclusive term for the mind and spirituality and is not limited ti the brain functions {manas}. Some construe this to this to mean the processes covered by Psychology; the Behavioral Sciences. I agree, if that includes parapsychology. Citta Niyama would include conditioning {sankharas / samskaras}, but also various powers of the mind that are beyond the limits of modern science. This covers the a-causal; such as synchronicity, as well the supra-causal and what appear to be miracles.
The concept of Panca Niyama, or Five Processes of Causality, provides a much more complete and rational view of cause and effect than blaming everything on Karma. Not everything that happens is due to Volitional Moral Causality. However, the Buddha did teach that every volitional action has a consequence.
Wholesome and unwholesome cetanas or motives will arise. It is up to us to either block & abandon them; or cultivate and maintain them. If we wish to change our karma for the better; then we can do the former. Meanwhile, we can endeavor to cultivate and maintain wholesome cetanas. That does not mean we get a free pass for bad deeds done in the past. Note that Angulimala, the gruesome serial killer that repented and reformed, still experienced his karmic retribution, albeit in a greatly lightened manner.
Also, dukkha or pain is inherent in Natural Law. Everyone experiences bad things that are just part of life and have nothing to do with karma. Moreover, changing our karma, via ethical conduct, is only entry level Dharma. The better part of Buddhism transcends karma, and is about Citta Bhavana; cultivation of one’s mind and spirituality.
Dwelling on the back roads by the rivers of our memories.
