They can by eating, like fish that never see the sunlight. Grass also contains Vitamin D.

Another theory is that many birds and Mammals get their vitamin D from excreated oils which then are ingested while grooming themselves.

I also found a study where cows where covered and produced way less vit D, so it's definitely skin as well. Just because an animal is covered in fur, doesn't mean 0 sunlight reaches the skin.

Brains of birds are also wired super differently. When it comes down to it's about the number of neurons and connections that make you smart. For very small animals, size can be a limiting factor for getting super smart, but the size of cells is also different between small and big animals. So, bigger brain doesn't necessarily mean more cells and computing power

do you know what you are talking about? nothing in this article states anything related to how vaccines works in us. as far as I remember plants have some genes that can detect pathogens and elicit some immune response, but it's not adaptive as in us mammals. pls don't spread misinformation after skimming over a Wikipedia article