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.

Golden Swamp Warbler

I spent a third consecutive weekend at Magee Marsh Wildlife Area and vicinity, and once again saw many interesting things. The action was much dissipated from the prior two visits, but there was still plenty of birds to go around. Lots of other good non-bird stuff, too, and I managed some interesting photos of some pretty cool and seldom-seen critters. Wait till you see the baby mink!

This is a female Prothonotary Warbler, Protonotaria citrea. Sometimes known as "Golden Swamp Warbler", a fitting moniker most would agree, after seeing the luminescent yellow of one of these swamp-dwellers glowing in the gloom of understory thickets. Even the females glow!

But there is no need to create fanciful alternate names - the real one is interesting enough. The scientific name tells the story. The specific epithet, citrea, means golden or yellowish-orange. Protonotaria, the source of the common name prothonotary, is a term of ancient origin referring to certain orders of clerks or notaries. It's use is often associated with the Catholic church, some of whom wore robes that included colors of bright yellow. So, for better or worse, our brilliant swamp flamer is named for a major religion.



The Magee boardwalk birds are probably the most famous Prothonotary Warblers in the world. Quite conspicuous in their coats of gold, and with the males announcing their presence with a loud, albeit unmusical, swee-swee-swee song, probably no one who has visited went away prothonotaryless. There were a number of years where the golden swampers were mostly absent from Magee, so it is great to see birds back and apparently going to nest.

I made the above video right next to the boardwalk, of a male who is either inspecting potential nest sites - this is our only eastern cavity-nesting warbler - or hunting food. Or both.

Without doubt, the Prothonotary Warblers were one of the real hits along the trail this month. Their bold, extroverted manners, coupled with outlandish good looks, thrilled many thousands of birders and they were undoubtedly lifers for a great many.