spring migration. The second, third, and fourth generations return to their northern locations in the United States and Canada in the spring. Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico, though they have a much lower migratory success rate than do wild monarchs (see section on captive-rearing below). Monarch overwintering sites have been discovered recently in Arizona. Monarchs from the eastern US generally migrate longer distances than monarchs from the western US. Since the 1800s, monarchs have spread throughout the world, and there are now many non-migratory populations globally. Flight speeds of adults are around . Interactions with predators In both caterpillar and butterfly form, monarchs are aposematic, warding off predators with a bright display of contrasting colors to warn potential predators of their undesirable taste and poisonous characteristics. One monarch researcher emphasizes that predation on eggs, larvae or adults is natural, since monarchs are part of the food chain, thus people should not take steps to kill predators of monarchs. Larvae feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepias species vary. Not all monarchs are unpalatable, but exhibit Batesian or automimics. Cardiac glycosides levels are higher in the abdomen and wings. Some predators can differentiate between these parts and consume the most palatable ones. Butterfly weed (A. tuberosa) lacks significant amounts of cardiac glycosides (cardenolides), but instead contains other types of toxic glycosides, including pregnanes. This difference may reduce the toxicity of monarchs whose larvae feed on that milkweed species, as a naturalist and others have reported that monarch caterpillars do not favor the plant. Some other milkweeds have similar characteristics. Types of predators While monarchs have a wide range of natural predators, none of these is suspected of causing harm to the overall population, or are the cause of the long-term declines in winter colony sizes. Several species of birds have acquired methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides (cardenolides). The black-backed oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The black-headed grosbeak, though, has developed an insensitivity to secondary plant poisons that allows it to ingest monarchs without vomiting. As a result, these orioles and grosbeaks periodically have high levels of cardenolides in their bodies, and they are forced to