Bird Predators

In the mid-1970s, scientists discovered Monarch overwintering roosts in Mexico. As they began to study these roosts, they found extraordinarily high mortality due to predation. Although researchers had reported some predation in California Monarch roosts before this, such high levels of predation spurred a number of studies investigating the species that preyed upon the Monarchs, ways those species avoided poisoning, the patterns of predation, and the impact of predation on Monarch populations.

Although many birds live in the same area as the Mexican roosts, only a few feed heavily on Monarchs. Of 37 bird species reported in the region, 25 never feed on Monarchs while another eight only rarely eat a Monarch. Two species, Scott's oriole and Stellar's jay, occasionally hunt Monarchs, which means that small numbers of birds visit the roosts irregularly to feed on Monarchs. The two remaining species, black-headed grosbeaks and black-backed orioles, are the main Monarch predators. These two species feed twice daily at the roosts in mixed flocks of five to at least 60 birds and annually consume several million Monarchs in the Mexican roosts.

Black-headed grosbeaks and black-backed orioles have very different ways of avoiding poisoning when they eat Monarchs. Grosbeaks, which eat the entire Monarch abdomen, are relatively insensitive to cardenolides and can tolerate moderate levels of these chemicals in their digestive tract. Orioles, on the other hand, vomit after consuming much smaller amounts of cardenolides. They avoid poisoning by not eating the cuticle, which is where Monarchs store cardenolides. Orioles slit open the body and strip out the soft insides.

Oriole and grosbeak predation follows several patterns. For example, grosbeaks eat fewer female Monarchs than orioles. Female Monarchs have, on average, 30% higher cardenolide concentrations than males. Females, therefore, may be more toxic than males. Since orioles avoid the cardenolide-laden cuticle, increasing concentrations may not affect them. On the other hand, females also contain more lipids (fats) than males and therefore may provide more nutrients per prey item than males. Scientists still have not determined the relative important of these two different forces (avoiding poisons and obtaining nutrients).

Other characteristics may affect the amount of predation too. The edge or periphery of the roost faces higher levels of predation than the center. Thus, small roosts, with a larger edge to interior ratio, face higher overall predation than larger roosts, which have a lower edge to interior ratio. Forest thinning, due to natural causes such as streambeds or human-induced causes such as selective logging or brush clearing, also increases predation. Thinning increases the surface area of the roost and makes the top side of the roost more accessible to predators.

Grosbeak and oriole predation causes more than 60% of Monarch mortality at many Mexican roosting sites, killing approximately 7 to 44% of the total population. At one 2.25 hectare colony, for example, birds ate an average of 15,000 butterflies daily and over 2 million for the season, which constituted 9% of the roost's population. Some scientists have speculated about the evolutionary impact of such high levels of predation. Since edges are more vulnerable to predation than interiors, Monarchs that overwinter in large colonies could be more likely to survive than those in smaller colonies; in large colonies, with their lower edge to interior ratio, a smaller percentage of the butterflies are vulnerable to edge predation. Early arrival at the roost would also be advantageous, since the center of the roost faces less predation risk than the edges. Avoiding predation could also be a force molding behaviors that help individuals regain a central position in the roost when they leave for water or nectar, or when they are blown down by storms.

Mice Predators

Five species of mice are known to be abundant near the Mexican overwintering roosts of Monarch butterflies. Of these five, only the scansorial black-eared mouse, Peromyscus melanotis, eats Monarchs. Several studies have investigated how P. melanotis avoids poisoning, the impact Monarch consumption has on the mouse, and the impact mouse predation has on the Monarch population.

An individual P. melanotis consumes an average of 37 Monarchs each night. They feed preferentially on easily accessible butterflies (near the ground) and on "wet" butterflies (dead butterflies that have not dried out). During the winter season, P. melanotis migrate to Monarch roosts where they set up residence and breed intensively. The population of mice in Monarch roosts generally do better than other mice of the same species whose territories are outside of the roosts; mice in roosts are larger, heavier, and reproduce more than mice outside the roosts. They are the only mammalian species scientists have found that have overcome Monarch chemical defenses enough to effectively exploit the dependable food source Monarch roosts represent.

Over the winter season, these mice may consume 4 to 5.7% of the total population of the Monarch colony. This translates into at least one million butterflies each winter in a 2.25 ha roost. This level of predation may also have helped mold Monarch behavior in winter roosts. For example, Monarchs generally crawl up vegetation. Moving up might help protect Monarchs from both freezing temperatures near the ground and from greater mouse predation. On a conservation note, this means that it is important to also protect the understory vegetation in winter roosts so that Monarchs can escape mouse predation more easily.