It’s two different explanations. The bee going from flower to flower is an example of fertilization and the bird incubating an egg is an example of gestation.
I have been asking me this question for a long time. What do birds and bees together have to do with reproduction? The only interaction they have is that some birds eat bees and that a bird can be stung by a bee. Now in my native German, the corresponding expression is “Bienchen und Blümchen”, “bees and flowers”. That makes sense, because you can teach kids about the concept of reproduction by telling them how bees pollinate flowers, and then you can take another step to explain how humans do it…