Sonogram image adapted from the Royal BC Museum.
The blue-headed vireo chants endless variations on a few, key ideas. The bird guides all speak of a kind of deliberation – one could just as well say thoughtfulness – in his delivery of short, robin-like phrases designed to carry a couple hundred yards through deep forest. His repertoire includes no more than twenty distinct phrases, but he seldom repeats a given sequence. Whether you choose to call this improvisation or shuffle play says more about you than it does about the vireo, who doesn’t care whether you view him as an artist or a machine.
The variations aren’t random, though: he consistently prefers some phrases to others. Some are drawn from a shared stock of blue-headed vireo folk material, so to speak, which varies from region to region. But each bird also has a few phrases that are unique to him. This vireo calling from a witch hazel branch on the first of May in a maturing chestnut oak-heath understory forest in central Pennsylvania sings a song never before heard in the three and a half billion years of life on earth. When he dies, it will die with him. Listen well.
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“Some males sing slowly when foraging between incubation bouts… Near nest with eggs, song often reduced to repetition of only 1-2 phrases,” says the most authoritative source (Ross D. James, “Blue-Headed Vireo,” in A. Poole and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, Cornell Laboratory of Ornithology and the Academy of Natural Sciences, No. 379 [1998]). This one chants four phrases, fixes his beady eye on me, then hops right into a beautiful little nest suspended from the branch of a mountain laurel bush some six and a half feet off the ground. It’s about the size of a teacup, the outer layer woven from what appear to be short pieces of bark from wild grapevine; dead leaves; white paper, presumably from some scrap of wind-blown litter; and gray paper from a hornet nest – probably that one over near the edge of the field that blew out of its tree during the high winds of March. Finer material used in the lining – dry grasses, rootlets, fungal mycelia and the like – protrudes slightly above the rim of the cup.
Like many nest builders, vireos are gifted at improvising with materials at hand. Spider web or the silk from last year’s caterpillars supplies the mortar that holds the whole construction together and fixes it to the branch. We tend to forget just how much silk persists in field and forest over winter. Several times this past winter and early spring I’ve been out when low sun was at just the right angle and was amazed by how many gleaming strands of gossamer still stretched between twigs and along the ground, even after the snow had melted.
These vireos have only been back from their winter sojourn in Central America or the Caribbean for three weeks, at most. They’ve wasted no time in pairing off, setting up house and, from all appearances, laying eggs and beginning to incubate. Like many neotropical migrant songbird species, they raise a single brood each season, though their early return gives them enough time to try re-nesting if a predator wipes out their first clutch. This is much more the exception than the rule, however, especially for the northern subspecies Vireo solitarius solitarius, which migrates the farthest.
And there are plenty of predators: crows, snakes, raccoons, gray squirrels – even normally vegetarian white-tailed deer have been observed eating songbird nestlings. This nest seems high enough to avoid the deer, and squirrel numbers are down following a couple of harsh winters and poor acorn crops. Then, too, the blue-headed vireo often nests “near small openings or edges of wetlands and lakes,” says Ross D. James, so it may have evolved more defenses toward edge-dwelling predators than other forest-interior specialists such as wood thrushes and scarlet tanagers, whose numbers are in decline throughout their range.
But the blue-headed vireo, too, likes large tracts of mostly unbroken, mature forest. This makes sense, because it’s the environment in which most of these neotropical migrant songbirds have evolved: “areas where extensive forest predominates . . . with trees that are middle-aged to mature, with high percent canopy closure (usually >75%), and where there is some (but not dense) understory of shrubs and saplings.” The blue-headed vireo displays more flexibility in habitat selection than some of the more specialized passerines, nesting in everything from northern conifer forests to dry oak forests like ours to the mixed mesophytic forests of the southern Appalachians. But the monograph warns that “Extensive clear-cutting is detrimental. Even partial clearing may be serious…”
The recent rediscovery of the ivory-billed woodpecker suggests that some species may be more resilient than we once supposed. But the rapid regeneration of southern swamp forests that were never completely denuded of all their oldest stands of bald cypress in the first place should not allow us to become lured into a false sense of security about the long-term survival of interior-forest specialists in the east. Around here, most forests were clearcut twice in the 19th century, back in the charcoal iron era, and at least once more in the early 20th century during the chemical wood era, before the petrochemical revolution gave the woods a break (while of course accelerating the degradation of nearly all ecosystems worldwide). But the reprieve thus granted forest-dwelling species is probably over. In the last couple of decades, economic and cultural forces have pushed the frontiers of year-round human settlement farther and farther into areas that were once thinly dotted with poor farms like ours or with hunting cabins. Deep woods habitats are disappearing.
A number of morbidly fascinating things happen when once-intact forests are fragmented by roads or subdivisions. As the proportion of edge to volume increases, secure refuges from artificially numerous habitat-generalist predators and barriers to the spread of non-native invasive species dwindle and disappear. While habitat edges or ecotones are often areas of concentrated biodiversity, they are very much a double-edged sword. The Great Eastern Forest once accounted for at least ninety percent of the land cover east of the tallgrass prairie; openings were small and temporary. The loss of naturally occurring forest openings in over-managed or too-young forests cannot fail to have negative impacts on species specializing in those kinds of environments. And the establishment of unnaturally long-term or permanent edges is thought to compromise essential functions of forest ecosystems through (for example) increased light and wind levels, which conspire to degrade the depth and quality of humus for hundreds of feet in from the edge. Keep in mind that the humus layer, where the base of the food chain is concentrated, contains the keys to the whole ecosystem. Air and water pollution – also greater along edges – can wreak havoc on soil chemistry, with probably dire consequences for the delicate balance of mostly unclassified microbial life.
The ripple effects from damage to the base of the food chain can take decades to register in the loss of what conservationists call “charismatic megafauna” – the mammals, birds, and occasionally reptiles and amphibians around which conservation campaigns tend to be built. DDT is still found in the tissues of many North American songbirds, and contrary to what one might suppose, levels are highest among non-migratory species in the United States, where use of the chemical was ubiquitous up until forty years ago. Who knows what its effects on songbird reproductive success might be? Acid precipitation leaches calcium from the soil, with negative consequences for land snails – one of our most numerous and biodiverse families of forest invertebrates. Ground-foraging bird species such as wood thrushes and ovenbirds eat a lot of snails, especially when they’re laying eggs and need the calcium. An on-going study in upstate New York has found a correlation between acid precipitation and nest failure among wood thrushes. Acid precipation is most severe and its effects most deleterious in ridgetop forests with unbuffered soils – which describes well over half of the intact forests in central Pennsylvania. And snails are also among the forest litter-dwelling organisms most likely to impacted by edge effects.
Some birds and mammals are highly adaptable to sudden change; many other organisms are not. The effects of habitat fragmentation can take a while to show up, because local and regional population collapses as a consequence of inbreeding depression don’t happen overnight. And how much time has to go by before anyone even notices and documents the change? The vast majority of scientific studies go on only as long as it takes a graduate student to earn a degree; institutional support is rarely forthcoming for the kind of long-range studies needed to compile comprehensive life histories of single species, let alone to disentangle ecosystem functions – a matter of guesswork for virtually every natural ecosystem on the planet. My mother recently told me about an expert on chickadees who once said that if she had halted her study after only two years, she would have ended up with different and in some cases directly opposite conclusions from those she arrived at after several decades of field observations and experiments.
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This kind of conservation message may seem a bit removed from the ordinary fodder here at Via Negativa, but it’s very close to my core motivation in launching this blog a year and a half ago. More than once I’ve quoted the ecologist’s mantra, attributed to Frank Engler: “Nature is not only more complex than we know, it’s more complex than we can know.” Knowing that we don’t know has always struck me as an extremely valuable insight, central to ancient and indigenous belief systems the world over. Shouldn’t the recognition of existential ignorance foster humility and reverence toward the objects of our unknowing? Indeed, as the epigraph from Rene Char says, “How can we live without the unknown before us?”
But as I said the other week in my post about the black snake, the most important mysteries are those that lurk within the scrim of the allegedly ordinary, the unique and unrepeatable details of particular beings and events. Hence the poetry, the ruminations, the daily cartoon, the attempts at fiction and photography; hence my fascination with the scatological and the bizarre. I refuse to leave anything out.
Now the digital camera gives me one more way to pile up evidence for what is, to me, a self-evident truth. This camera isn’t very good at close-ups or depth of field. A still photo tells you little about context and motion and nothing at all about sound or odor. The wind rocked the branch; a ten-second rain shower pattered down. When the sun came out, the vireo’s white-ringed black eye seemed more beady than ever as his head stretched and swiveled in all directions. I walked slowly within flash range, then even closer, snapping away. He never left the nest. Hours later, when I returned with my mother, we stopped short some fifteen feet away, concerned that our scent trail could lead predators to the nest.
We felt privileged to have seen this much, the male bird having seemingly showed me his nest for reasons that are difficult to imagine. The Birds of North America monograph claims that the species is “very sensitive to close human attention at time of pairing and early nest-building; female readily abandons nest and even [her] mate. However, may nest in or near campground with unobservant traffic. Much more tolerant once eggs are laid. Sensitive again when large young are in nest…” “Tolerant” hardly begins to describe that male’s behavior. Mom says a hooded warbler did the same thing to her last year, in another part of the property: sang until she noticed him, then hopped into his nearby nest.
I tied a few ribbons of surveyor’s tape on the way back to the old woods road we use for a walking trail, one of several such – relics from the original clearcutting of the mountain circa 1815. I’ll leave it to the resident naturalist’s discretion whether and how often she wants to revisit this nest. For my part, I’ve seen enough. If before I was still a bit fuzzy on the difference between the blue-headed and red-eyed vireo songs, now I wonder how I ever could have confused them. Minimal as it may be, the importance of this kind of knowledge should not be underestimated: it’s what makes us feel at home in a place, knowing the names of our neighbors and some of their habits.
Later in the afternoon I sat out on my porch for a few minutes and listened to another blue-headed vireo singing up in the woods, probably defending the next territory down from the one that I’d watched. Maybe by the end of the summer I’ll learn to tell them apart, these two singers. And maybe I’ll learn something from the effort. A few ideas, endlessly repeated in endlessly novel ways: it’s no more or less than what I’ve always strived for in my own attempts to tell the world what’s what.
