They migrate in colonies. Birds avoid monarchs because the caterpillars are born on milkweed plants. The caterpillars eat the milkweed which is poisonous to the birds. There is another butterfly that copies the monarch, but does not eat milkweed plants. The birds still avoid the Viceroy butterfly just because it looks like a monarch.
Some scientists think monarchs moved more north as the milkweed plant moved north.
Monarchs can not be in cold weather. Monarchs need the right temperature and humidity. They need to be protected from the rain. Monarchs find everything they need in the Mexican mountains. That is where they go for the winter.