yes
They are not proper nouns. They would not be capitalized in a sentence.
No it shouldn't be capitalized.
Yes, Siamese would be capitalized.
No, they are not capitalized, just as math and geography would not be capitalized.
No, the word "century" is not capitalized unless it begins a sentence. So "thirteenth century" would not be capitalized in the middle of a sentence.
You would only capitalize pelican if it was at the beginning of the sentence. Pelican is not a proper noun, so it would not be capitalized.
No, "pigeon" is not typically capitalized in a sentence unless it is part of a proper noun or at the beginning of a sentence.
No, "sister" is not capitalized in the middle of a sentence unless it is used as part of the name or title that is capitalized. For example, in "I have a younger sister," "sister" would not be capitalized.
Euthanasia is illegal in the United States.
No, but if you named the purple amethyst Shelly, then it would be capitalized.
No. It is only if it is at the start of a sentence that it would be.
To prevent his sick dog from suffering anymore, he wanted his dog to have euthanasia.