When chatting with ecologists about moth caterpillars, it is surprisingly hard not to get hungry.
“I always talk about them as sausages: a very thin wrapper filled with good things,” says University of Delaware entomology professor Doug Tallamy, whose research has shown that native plants provide more bugs for birds to eat. Morgan Tingley, a University of California, Los Angeles ornithologist, proposes another culinary comparison: “Caterpillars are like the bread of the diet of baby birds. They’re a staple.” Ohio-based naturalist Jim McCormac splits the difference. “They’re nature’s hotdog,” he says.
Human appetites aside, entomologists and ornithologists agree that caterpillars—the larval stage of both moths and butterflies—anchor the diets of many birds worldwide, especially those that dine in temperate forests and foliage. They provide nestlings with plentiful pillows of fats and proteins, and some offer a boost of feather-pigmenting carotenoids. Yet compared to their beloved butterfly brethren, moths are far more edible, abundant, diverse, and overlooked.
Cryptic by day, moth caterpillars might burrow in the ground or disguise themselves as twigs. Some larvae of millimeters-long moths even snuggle, as if in bedsheets, between a leaf’s outer layers. Once the sun sets, twilight- and night-flying moths of all ages emerge, only to face an unrelenting onslaught of predators including bats, parasitic wasps, and, of course, a number of birds. Then, the gorge-fest begins.
A single clutch of Carolina Chickadee chicks can feast on upward of 9,000 caterpillars in the weeks between hatching and taking flight. When in 1980 biologists examined Shining Cuckoo innards in New Zealand, about half of the 23 insects found per stomach, on average, were caterpillars. In North America, more than 100 species depend on caterpillars part of their diet, and larvae provide a majority of the diets for birds like the Tennessee Warbler, Red-eyed Vireo, and Rose-breasted Grosbeak. Consider that both Yellow-billed and Black-billed Cuckoos have the distinct ability to ingest very-hairy tent caterpillars—up to 100 in a single, hungry sitting. In Europe and Asia, Great Tits prefer winter and oak leafroller moth caterpillars. (None of this is to forget the smaller subset of birds, such as American Redstarts, that feed on adult moths and other insects.)
In fact, moth catepillars are so coveted as a food source that they “live on a knife’s edge” where the survival of one or two adults out of a brood of 200 or 300 eggs is a biological accomplishment, writes University of Connecticut ecologist and lepidopterist David Wagner. Tallamy agrees: “The mortality is fantastic,” he says. “It makes the world go ‘round.”
Under those precarious survival conditions, however, any number of environmental changes can disrupt moth populations. Overall, scientists believe a variety of factors—deforestation, light pollution, habitat degradation, insecticide use, and climate change among them—may share some blame for growing evidence of moth declines. For example, a study in the Netherlands reported about 80 percent of day-flying moth species experienced shrinking ranges over a 20-year period. In 2014, an analysis of 11 million British records documented a similar trend.
There’s less research in North America, says Tingley, but moths aren’t likely faring much better. In 2017, researchers discovered the decline of eight species of hawk moths in the Northeast, and the disappearance of two more from parts of their ranges. Soon after Tallamy and his colleagues demonstrated that hedges dominated by invasive plant species hosted about one-tenth the number of caterpillars compared to all-native hedges.
“There’s no question that insects are declining. We don’t know if it’s an apocalypse or Armageddon. Those are dangerous words,” Wagner says, referring to much-debated reports of massive, global drops in insect abundance and diversity. “The decay occurs over decades. You have to be halfway through your lifetime to notice something has changed. It’s been an almost imperceptible malaise.”
It is also hard to definitively connect moth or butterfly declines to avian ones. Researchers have noted, however, that insectivorous birds that eat adult moths, such as swallows, swifts, nightjars, and flycatchers, have experienced some of the most significant losses of all bird groups in North America since 1970. “It’s death by a thousand cuts—so it’s really hard to pinpoint any specific cut,” Wagner says. “There would appear to be a link, but we don’t have unequivocal data. We don’t have incontrovertible evidence.”
Researchers are beginning to tease out the relationship between the loss of moths and harm to birds that eat them. Canadian biologists recently compared the claw and feather tissues of Eastern Whip-poor-wills, whose diet is about 60 percent moths, with museum specimens from 1880 to 2005. They discovered evidence the bird could be suffering from a shrinking supply of prey over time. In 2018, Hungarian researchers reported Great Tits in urban areas where caterpillars are less abundant laid smaller clutches, experienced more frequent nestling mortality from starvation, and reared fewer offspring to fledging age.
In the United States, UCLA's Tingley is teaming up with entomologists to combine decades of data from birders with harder-to-get information on insect populations. One of his collaborators, avian ecologist Allen Hurlbert at the University of North Carolina at Chapel Hill, runs the program Caterpillars Count!, a community science project that tracks the availability of this source of bird food. But it’s too early, they say, to draw any conclusions.
Wellesley College professor emeritus Nicholas Rodenhouse, who has studied Black-throated Blue Warblers since 1982, says the species has continued to reproduce successfully in New Hampshire’s Hubbard Brook Experimental Forest, despite the site’s decades-long downward trend in insect biomass, including caterpillars. “In our area, they’re holding up. How can that happen?” he says. “Don’t underestimate how incredibly flexible birds are. They’ll eat all sorts of stuff. I think of them as little vacuum cleaners flying through the forest.”