Red-eyed Vireo harvests tent caterpillar nest silk

Apologies for the lack of posts of late. I just returned from my annual foray to the wilds of West Virginia to participate in the New River Birding and Nature Festival. While there, I led trips for six days, then Shauna came down and we had an epic trip into the Monongahela National Forest yesterday. There we made many photographs of a variety of interesting organisms, some of which will probably surface here later.

FYI: The New River Birding and Nature Festival takes place late April/early May, and this was its 23rd year. This region of southern West Virginia is exceedingly rich in biodiversity, including birds, and our trips fan out to a variety of habitats. One of the main targets for many people is Swainson's Warbler (I'll try to make a separate post about that later), and the Fayetteville area (where we are based) is an epicenter for them. See more festival details HERE.

While leading the trips for the festival, I take next to no images and only carry my iPhone and my Canon R5 with a 24-105mm lens. The latter of which is used primarily to get group shots, and maybe some habitat images. As a guide, one must remain on point the entire time on the field, and prioritize helping group members get on birds, and learn more about natural history. There isn't time for taking images, although sometimes I experience some inner agony when we see amazing things and I cannot commemorate them photographically. But I get as much enjoyment out of seeing things and learning more about them as I do creating images.

With that in mind, the following photograph is shared courtesy of one of our participants on last Saturday's trip, Carisa Collins. She uses a Nikon point & shoot with an amazing reach and does a superb job of capturing images of various bird species that we see. The story and photo follow.

Photo courtesy Carisa Collins

Last Saturday, Geoff Heeter and I led a trip into the Summit Bechtel Reserve, a massive scout camp property near Fayetteville, West Virginia. We saw many interesting birds, but a personal highlight was seeing this female Red-eyed Vireo (females select nest sites and build the nest) collecting silk from an Eastern Tent Caterpillar (Malacosoma americanum) nest.

When we saw the bird approaching the nest, I thought the vireo was going to raid it for caterpillars (they are the bulk of a Red-eyed Vireo's diet). That would have been interesting, as the only birds that I know of that routinely raid these nests are cuckoos and Baltimore Orioles. Tent caterpillars are heavily beset with spines that inhibit most birds from eating them, and when in their silken nests are well protected from avian predation. Big cuckoos (both Black-billed and Yellow-billed) can rip into the nests and ravage the occupants. While a cuckoo may swipe a captured caterpillar back and forth across a branch in an attempt to remove some of the spiny hairs, they do end up swallowing many, and it is said that dissections of cuckoo specimens have revealed that the stomach linings are liberally fuzzed with hairs that penetrated the lining. I have heard that cuckoos can essentially regurgitate the stomach lining if it becomes too choked with tent caterpillar spines, and can regrow a new one, but haven't verified that.

Less well known is that Baltimore Orioles also feed on tent caterpillars. I made a post back in 2014 about this, with plenty of documentation. See that post RIGHT HERE.

Anyway, the vireo that is the protagonist of this story was only interested in the nest's silk, and she had to struggle to separate the wiry cable-like material. She'd tug and tug and tug before successfully separating a tuft of silk. It'll be used to bond her intricate cuplike nest.

Many people despise tent caterpillars - a native moth - because of the nests which they consider unsightly. That's a very shallow uninformed viewpoint. It is a native moth, the nests do not kill the host tree (cherries), and they spawn scores of insect species that prey on the caterpillars. Many of those insects, many of which are parasitoid wasps, in turn become food for other animals. The adult moths serve as pollinators and are eaten by other animals. Bats, who prey primarily on moths, surely eat many  tentworm moths. And now we know that Red-eyed Vireos - which winter in South America - utilize their nests. Eastern Tent Moths are a major keystone species with far-reaching importance when bird predators are factored in. Both Black-billed and Yellow-billed cuckoos, and Red-eyed Vireo, winter deep into South America, and Baltimore Orioles winter throughout much of Central America, northern South America, and the Caribbean islands. The lowly eastern tent caterpillar is an important part of their life cycle.

Camo caterpillars and the birds that eat them

 

A Red-eyed Vireo (Vireo olivaceus) in a characteristic hunting posture. The bird is searching for its major prey, the caterpillars of moths. It has its head cocked upwards, searching the lower surfaces of the leaves above, as that's where many caterpillars hide during the day. Lepidopteran larvae - and those are OVERWHELMING moth caterpillars (not butterfly caterpillars) - are the vireo's major food source. During a long summer day, the roughly 130 million Red-eyed Vireos breeding in the U.S. and Canada consume something on the order of 4 billion caterpillars. Hard to believe but based on what is known of their foraging rate and dominant prey items, that's how the numbers shake out.

A Prothonotary Warbler (Protonotaria citrea) with a freshly harvested caterpillar (unsure of species). Almost all of the 38 warbler species that breed in eastern North America eats scads of caterpillars, and so do most groups of songbirds. So do some nonpasserine species, most notably the cuckoos. Many of these birds are migrants that winter in more southerly haunts, often in Central or South America, and migrate northward during the breeding season to exploit the eastern deciduous forest region's vast bounty of caterpillars. As this is crop is only seasonally available, many of these bird species must travel back to warmer climes for the winter - where they undoubtedly also consume great numbers of caterpillars.

If you are a caterpillar, you don't want to be grabbed and eaten by a bird. Evolutionary warfare between caterpillars and birds is probably the main driver of some incredible caterpillar camouflage. If you are a plant-eating caterpillar and can become ever more like the appearance of the plants that you feed on, the odds of surviving surely goes up. Birds are endowed with incredibly keen eyesight and are quite adept at spotting larvae. They still find plenty, and between the avifauna and all of the other predators that take out caterpillars, the survivorship can be as low as 1% in some moth species. So, every advantage that a caterpillar can get to thwart predators becomes very important.

Following are some photos, most from recent forays, that showcase examples of incredible botanical (and gall) mimicry by members of the larval crowd.

Late summer and fall foliage become dappled with color as the chlorophyll fades and leaf tissue begins dying. This plump Blinded Sphinx Moth (Paonias excaecata) caterpillar displays reddish patches that mimic autumnal foliage, in this case Hop-hornbeam (Ostrya virginiana).

This is a sister species of the caterpillar above, the Small-eyed Sphinx (Paonias myops), which also sports rusty dots that resemble necrotic leaf tissue.

This is a personal favorite of the leaf-edge mimicry crowd, a Double-toothed Prominent (Nerice bidentata). It is a specialist of elms, and in our part of the world that's almost entirely American Elm (Ulmus americana) and Red Elm (U. rubra). We find them on both of this species with roughly equal frequency. The back of the caterpillar is scalloped, much like the rough marginal serrations of elm leaves. As it eats into the leaf, the caterpillar becomes the leaf.

This is another elm specialist (mostly at least, it is also said to eat basswood), and its moniker is apropos: Elm Sphinx (Ceratomia amyntor). When at rest on the underside of an elm leaf, as above, the caterpillar typically aligns itself with the prominent midrib of the leaf. The caterpillar also has a raised stripe down the center of its back. The cat's lateral lines are arrayed at acute angles to its pseudo midrib and mirror the angled venation of the elm leaf. In effect, the caterpillar becomes a continuation of the leaf's veins.

Few can match the Checker-fringed Prominent (Schizura ipomoeae) caterpillar's ability to become one with the leaf. This one (the head is to the bottom) is consuming an old dogwood leaf and as is typical, the caterpillar is eating inwards from the leaf's edge. In essence, it becomes the leaf and spotting one of these can be quite difficult even when it is right in front of you. Sharp-eyed botanists might note the cat's scientific epithet: ipomoeae. That references a genus of morning-glories and is probably a misnomer, as this caterpillar probably never eats those plants.

Few do twig mimicry better than the caterpillar of the Dot-lined White Moth (Artace cribaria). They are virtually impossible to see as they lay plastered to twigs, despite being a hefty cat.

Another brilliant example of twig mimicry is the Yellow Slant-line Moth caterpillar (Tetracis crocallata). This is one of the inchworms in the massive Geometrid family, and many inchworms are twig mimics. When not feeding, they attach themselves to a twig of similar diameter and color, and often adopt an acute outward angle. In the case of this species, the head is jagged and pale yellow, exactly as a broken-off branch would appear.

A Redbud (Cercis canadensis) leaf, with what appears to be a raised reddish gall. Such leaf galls are commonplace, and can be formed by many organisms: bacteria, fungi, aphids, midges, wasps and others. Birds don't eat them, insofar as I know.

But wait! Closer examination shows that our leaf "gall" is not a gall at all. It is a Red-crossed Button Slug caterpillar (Tortricidia pallida). Slug moth caterpillars are fascinating as a rule, as are the "cute" little moths that they become. As caterpillars, many species appear to mimic the look of leaf anomalies such as galls, and they can be hard to spot.

This is just a tiny tip of the lepidopteran iceberg when it comes to caterpillar mimicry, much of which is presumably driven by sharp-eyed birds that attempt to feed ravenously on the caterpillar crowd.