Monarchs are (and always have been) susceptible to predation from mammals and birds as well as insects, parasites, and parasitoids. Even without facing the contemporary human-created perils of climate change, habitat loss, and pesticides, a monarch egg has only a 5 percent chance of surviving to the adult stage. Why is survival so low, even in the best of natural circumstances?

The largest losses occur at the egg stage, due to predation by insects, desiccation, or dislodgment from milkweed leaves by rain and wind. Monarch caterpillars and chrysalises — although somewhat less vulnerable — are also preyed on by invertebrate predators and parasitoids.

However, for centuries the species has flourished in spite of these vulnerabilities, offset by the ability of a single female to lay 300-500 eggs.

Milkweed is the monarch’s secret weapon. Monarch butterflies lay their eggs on milkweed plants. When the caterpillars hatch, they eat the milkweed. The milkweed plant contains toxins (cardenolides) that taste bad that can kill any animal that preys on the milkweed. Predators are discouraged from trying to consume monarchs because they taste bad as a result of these toxins.

And the vivid markings of the monarch caterpillar and butterfly are thought to be aposematic (or “warning!”) coloration, clear signs for predators not to eat them because they have sequestered away plant poisons.

This system works well for defense against vertebrates, notably birds and small rodents. When these animals consume a monarch caterpillar, they ingest the whole body and thus receive all the toxic cardenolides. In contrast, insects are capable of selectively eating the parts of the caterpillar that contain the least milkweed sap. This explains why insects are the more powerful predators.

Vertebrate Predators

Most vertebrate predators — birds, rodents and other small mammals — pose relatively little threat to Western monarchs. ·One reason for this is that most of them are relatively vulnerable to the poisonous chemicals that the monarchs have ingested from eating milkweed.

Some vertebrate predators have evolved to become immune to these toxins. Recent research indicates that successful monarch predators, such as the black-headed grosbeak, which typically overwinter in Mexico, have evolved mutations in one or more copies of a gene for the sodium-potassium pump. These are the same mutations as have been acquired by monarch butterflies, mutations that are critical to the monarch’s ability to eat milkweed without succumbing to its toxins.

In Marin, vertebrate predators can sometimes be problematic at overwintering sites along the coast. Observers at these sites report seeing monarchs occasionally facing attacks from jays, hawks, squirrels and mice.

The most effective way to offset predation in overwintering sites is by providing ground cover, understory plants, and roosting trees where the monarchs can take cover from harmful vertebrates.