This refers either to 1) crows, a kind of bird that eats mostly grain, or 2) ravens, a kind of bird that eats the flesh of dead animals. Jesus' audience would have considered the ravens worthless since the Jewish people could not eat these types of birds.
This is an exclamation, not a question. Jesus emphasizes the fact that people are much more valuable to God then birds. (See: [[rc://en/ta/man/translate/figs-exclamations]])
Jesus uses a question to teach his disciples. AT: "None of you can make your life any longer by being anxious!" (See: [[rc://en/ta/man/translate/figs-rquestion]])
# add a cubit to his lifespan
This is a metaphor because a cubit is a measure of length, rather than of time. The image is of a person's life stretched out as if it were a board, a rope, or some other physical object. (See: [[rc://en/ta/man/translate/figs-metaphor]])
Jesus uses another question to teach his disciples. AT: "Since you cannot do even this small thing, you should not worry about the other things." (See: [[rc://en/ta/man/translate/figs-rquestion]])