The Hummingbird and the Woodpecker - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 14. The Hummingbird and the Woodpecker

14 April 2006. I AM UP AT SIX AM, RISING WITH THE SUN and beating it only slightly. Perfect timing, because now is when the action is. The sapsuckers have just returned, and one has found the aluminum ladder I had put up only yesterday afternoon by the shed at our house in Vermont. He is drumming on it—“rat-tatatattat-tat” over and over again. Three more male sapsuckers came to the ladder as well and I wondered if they would join in, but they chased each other instead and then all three left. One came back to the ladder several times later to drum some more. A unique bird, this species. It’s the only one of the six local woodpeckers that has a non-uniform drumming rhythm, and that taps trees for sap.

17 June 2005. I’m on a platform of boards I have built about twenty feet up, between some close-spaced young red maple trees near my camp in Maine. I’m facing a single big birch tree that has four active sapsucker licks. The tree has just leafed out, although many of the other forest trees are still bare. There is a light rain, and temperatures are in the mid-fifties Fahrenheit.

Sapsucker licks are hubs of life in these woods at this time, and I want to take the pulse of this life. I hope to see surprises, even as I want to learn the routine. Today I’ve already established that there are at least two sapsuckers coming to this one tree. They regularly come from the same direction, fly to a tap, touch the sap hole with the tip of the bill, extend the tongue, and lick sap as they vibrate the head rapidly. Numerous ruby-throated hummingbirds come and go, and some of them stay and aggressively defend the site against others that come near. One female perched near me and about ten feet from a lick. She jerked her head from side to side, steadily, like a metronome, once per second. I knew she was getting ready for another feed at a lick when she turned in that direction, stretched, and then zipped over to it and hovered to feed for about thirty seconds before zooming off into the woods. Another then took her place. In the afternoon, a sharp-shinned hawk shot past me like an arrow through still leafless red maple branches. It landed on a limb next to a lick; nervously turned its head here and there, probably looking for prey; and then zoomed on as quickly as it had come.

22 August 2005. I could not have chosen a better day to be in the woods, up on the same sapsucker lick viewing platform. It’s seven-thirty AM and the sun is up. I’m twenty feet up, under the leafy canopy of young red maples. I have brought along a hot cup of coffee from the cabin, and sipping leisurely, face the thick birch tree with the woodpeckers’ licks. A few red maple leaves around me show the first twinges of red, and the field next to me is ablaze with goldenrod bloom. A flock of about a dozen chickadees come sauntering by. After they pass I hear warblers cheeping in the distance. Warblers sang lustily months earlier when they were separated into their respective territories, each in its own specific habitat. Now, all is different. A loose flock of them soon drifts in, and it includes at least eight species: one chestnut-sided warbler, several black-and-white warblers, several blackburnian warblers, several ovenbirds, many Canada warblers, many black-throated blue warblers, a couple of black-throated green warblers, and several redstarts.

Along with the warblers are at least one wood thrush, one least flycatcher, and a phoebe. They stayed busily foraging all around me, for about twenty minutes. Many of them came within several feet of my face.

Six licks (each with dozens of sapsucker holes) had been heavily used for almost three months. But in one hour of watching I saw only one hummingbird come for a sip. This female (or juvenile) flew to all of the six taps but stopped only briefly, as though inspecting them and finding them dry. Except for one or two black-throated blue warblers, which hovered twice in front of a sapsucker lick but did not contact it, none of the warblers paid any attention to them. Only one sapsucker came, a juvenile from this year’s hatch. It perched about six feet from a lick for a full twenty minutes; moved its head occasionally to scan around; then stretched, preened for another ten minutes, and finally briefly inspected two licks before departing.

Twice an admiral butterfly flew by the licks without landing, and the juvenile sapsucker made a pass at it but missed. A knot of a dozen bald-faced hornets aggregated at one spot at one lick only. Perhaps they had located the one last bit of sap flow. A downy woodpecker, which had previously sometimes used the same lick, came within a foot of it, looked at it, and then flew back into the forest of mostly maples, pines, and firs. It seemed to me as though the steady customers at this lick site were coming to check on it, but not finding much of anything.

I come down from my perch and then climb up the birch to inspect the licks. As expected, they are indeed dry now although the woods are still lush and green. But what a difference from what they were like a few weeks earlier! There is no more birdsong. Even the sapsuckers, who were noisily drumming all around this lick when they first came in mid-April, and who continued vigorously into late June and July when their young fledged, have now ceased. The summer is ending and life is clearly on a much more leisurely schedule. The birds may already be fattening, preparing to leave the summer world behind them.

LIFE FEEDS ON OTHER LIFE, AND IT’S NOT ALL ABOUT FLIES sucking blood. Indeed, most of the interesting and—to us—uplifting nature stories emerge from other feeding relationships. Even as I write on 2 July 2007, the Amelanchier (Juneberry) tree in front of my office is being visited by robins, cedar waxwings, purple finches, rose-breasted grosbeaks, a catbird, and a veery. They are feeding on the now ripening (but not yet ripe) berries, and will be spreading seeds in all directions to potentially “plant” more trees. Whether or not we approve of any feeding relationship generally depends on whether we are on the giving or the receiving end. The ruby-throated hummingbird that brightens our summer days is at both the giving and the receiving end of feeding relationships: the receiving end is from us and sapsuckers, and the giving end is the pollination of plants in Central America.

There are at least eighty-six species of hummingbirds in Brazil alone, of 343 species documented so far. America is the hummingbirds’ home, and the South American tropics are the heartland of these jewels of the bird world. Here, in New England, we are graced by only one species, the ruby-throated hummingbird, Archilochus colubris. It is a suburban favorite that is easily attracted to a bottle of plain sugar water marked with a red artificial flower.

Like all hummingbirds, the ruby-throat belongs to a family exquisitely adapted to feed on the nectar of flowers, and many flowers have coadapted to be pollinated by them; one depends on the other. “Hummingbird flowers” have a long tubular neck (corolla) that excludes “nectar thieves”—those animals that feed from these flowers but do not pollinate them. Hummingbird flowers are commonly red. In contrast, the flowers pollinated by sphinx moths—the nocturnal analogues of hummingbirds—are white and strongly scented, to be more easily located by prospective pollinators.

Many hummingbird species are, in turn, adapted to specific kinds of flowers. Even the length and curvature of their bills fit specific flowers (and exclude others). But the ruby-throated hummingbird is, in many ways, one of a kind. It is the only hummingbird that has extended its range into the coniferous forests of northern Maine and Canada, in areas where all summer long there are only small white nectarless flowers on the mossy ground, and no red flowers at all. Farther south I had been astounded to see ruby-throats when there was still snow on the ground, long before any flowers opened, and before there were any leaves on the trees. Once in April when there was still snow on the ground, a hummingbird appeared as if out of nowhere and hovered briefly in front of my face. A friend told me of seeing another one hovering around the head of a pileated woodpecker. I had been wearing a red hat. The woodpecker has bright red feathers on its head. I suspect that these hummingbirds had recently come back from their tropical wintering grounds, where they had been feeding at red flowers. On coming north they were still responding to the same signal that had meant food. But why and how did and could they have risked leaving their tropical paradise with red flowers, to come so far north at a time when there is no red and there are no flowers with nectar?

A male ruby-throat weighs about one-tenth of an ounce, only slightly more than a penny (about 0.08 ounce) and several times lighter than many large moth caterpillars. Its heart and wings beat at twenty-one and sixty times per second, respectively, while it flies north on a journey of about 2,000 miles and then flies the same journey south in the fall. At any given moment it is within hours of starvation, as it needs to consume about twice its body weight of food per day. No other hummingbird attempts what would appear to be a very risky journey, to a destination often devoid of nectar-bearing flowers. When hummingbirds are not traveling, their body temperature often drops until they are torpid, to conserve energy when they can’t forage, as at night. They prepare for their long nocturnal fast by fattening up during the day. During migration, they replenish in the morning, leaving the middle of the day for traveling. Indeed, at Hawk Mountain in Pennsylvania in the fall, most of the migrant hummingbirds come through at midday (Willimont et al. 1988). But apparently they change their strategy on another leg of their migratory route, when they cross the Gulf of Mexico.

The Gulf of Mexico presents 520 miles of open water that must be crossed at one go and without refueling. At their top sustained flight speed of about thirty miles per hour, they face a nonstop flight of about seventeen hours. And here, before tackling that distance, the hummers depart from Alabama in late afternoon for a night flight directly across the gulf (Robinson et al. 1996). In the spring the birds returning to Alabama (at the banding station at Fort Morgan) arrive in the dark of night (Sargent 1999). Apparently some of them also take the longer route along the coast of Texas, where they could migrate by short hops and presumably refuel. Do they decide on one or the other option on the basis of their fat reserves? Do they know what they are up against before they take off over the open water of the gulf?

Although migration is hazardous, it can’t be excessively so for the hummingbirds, since they have one of the lowest reproductive rates of any northern bird migrants. They raise only one clutch of two young per year (perhaps because the female alone does all the work). By contrast, a pair of northern warblers will raise four to five young in a clutch, and a pair of golden-crowned kinglets will raise eight to twelve chicks per brood and nest twice per summer. Since on average these bird populations are stable over time, the number of offspring they raise provides a measure of their mortality rates; the hummingbirds must therefore have a relatively low death rate. We know they come north to build their tiny nest cups of lichens held together by spiderwebs, where the female rears her clutch of two young. But why not do it in the south? Why not stay in their ancestral home along with most others of their group? There are many theories, but no answers. There are, however, some answers regarding how they get by once they reach the north.

The male hummingbirds are the vanguard in the northward migration, as is true of most other bird migrants (Stichter 2004). The standard explanation for this phenomenon is that the males compete to establish territories in order to attract better females. It sounds reasonable. But then in the migration south at the end of the breeding season, the males again precede the females and the young of the current year.

For many years I thought that the hummingbirds I had seen in the northern Maine spruce fir woods in May must have returned much too early by mistake. Later I learned they were right on schedule. Their timing is synchronized with the return of a woodpecker, the yellow-bellied sapsucker, Sphyrapicus varius, which is their main food provider then, and all summer long.

The sapsucker is one of the most visually striking of birds. Its bold black and white markings contrast with a scarlet crown, a scarlet throat in males, and a soft lemon yellow-tinged belly. Sapsuckers differ from other woodpeckers in not having a long pointed tongue with barbs at the end. Instead, their tongue is much shorter, and the end is brushlike—an adaptation like the hummingbirds’ tongue, which is also for sweeping up liquid as with a wick.

Like hummers, the sapsucker males precede the females north, and as soon as the males arrive the woods around our house resound with their calls and their drumming. Females come a few days later, and in two weeks nest holes are drilled and egg laying begins. One of these early-arriving females hit our window and was killed, so I examined and sketched her.

These woodpeckers do not excavate insect larvae from wood as other woodpeckers do, and they don’t need the long tongues of other wood-peckers to explore tunnels made by longhorned beetle larvae. Nor do they excavate any hardwood, except their nest holes. They usually choose poplars that are softened by fungus (Fomes igniarius var. populinus) rotting them on the inside.

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Fig. 27. A sapsucker killed by flying into a window in April 2006. It was an immature female, as shown by the ovary with only undeveloped eggs. For woodpeckers, this species has an unusually short tongue, and relative to resident birds it has very long wings (like most migrants).

Adult sapsuckers eat sugar at their licks, as well as ants and other insects that also come for the sugar. They make holes through the bark, and then use their brushy tongue to lap up the sap. The most conspicuous sap licks are those on birch trees. The whole tree is commonly ringed by tiers of holes, which are visible from afar. Every year, for several years, the birds make new taps directly above and to the side of the old ones. Then the tree dies, and the birds attack another tree. On my hill in Maine I found half a dozen sap stations on birches where sapsuckers were active from late May through most of the summer. Although there were hundreds of holes in the bark at each of six lick sites on one large white birch tree that I examined, only a the topmost holes at any one lick site yielded sap—all the lower ones were dry. I lapped the sap and it tasted sweet. The sugar concentration (measured with a brewer’s refractometer) was 17 to 18 percent—similar to that of concentrated nectar.

Sapsuckers come back as early as the first week of April, and it seemed a mystery to me why birch, which produces sweet sap and seemed to be so much preferred in the summer, was not visited in the spring. Nor were the birds at any other sap licks that I could see, until at least a month later. What did they feed on when they came back before summer got started?

It was not until the spring of 2006 that I figured out the woodpeckers’ solution for obtaining food on their early return from migration. I had underestimated the birds’ sophistication. That spring I deliberately followed sapsuckers to see what they do. To my great surprise they were tapping sugar maple trees! It had always seemed to me that sugar maples should be an obvious choice for them. But I had not seen on these trees the patchwork of holes that is typical of their work on birches, which is what I was looking for.

It turned out that on sugar maples they punched only tiny holes, here and there. These holes were almost invisible, except on close inspection, and they quickly healed. However, in early spring, and only then, any tiny hole in a sugar maple “bled” sap profusely. Any maple tree I chose to puncture with the tip of my knife yielded the same result—huge droplets of sap welling up within seconds. But damage to the bark alone had no effect; it was necessary to puncture through it, if only slightly into the wood (i.e., xylem). Here was the answer: the woodpeckers were indeed tapping maples (and other trees), but the effect on the trees was so slight then that it left almost no visible marks. Then, in summer, when the woodpeckers switched over to birches, they made huge patches of holes that eventually killed the trees. (I later found that they only preferred to tap the phloem sap from birches, rather than maples; I eventually found a phloem tap each on a sugar maple and a red maple tree that were used in the summer. Both looked identical to the phloem taps that are very common on birches.)

The reason for the bird’s change of behavior from spring to summer depended on tree physiology and, unlike the sapsuckers, I had not considered the fluid mechanics of the trees’ “plumbing.” The early-arriving sapsuckers were accessing the sugar maple sap that was going up in the wood (xylem) of the tree, when it is apparently under pressure and comes out through any tiny puncture. Later in the season—and only then—when the xylem flow is low, and when our own traditional maple sugaring stops, they start taking sap from phloem in birches. The phloem, a living layer of the inner bark, transports sucrose and other products of the leaves down to the trunk and the roots. So the birches did not yield the rich phloem sap until they had put on their leaves, when they were transporting nutrients down.

On a birch, the sapsuckers make a large aggregation of wounds in a very distinctive pattern. Each individual wound is a somewhat square quarter-inch to half-inch bare patch where the bark is removed down to but never into the wood. Series of these bare patches are placed neatly one above the other in vertical rows, and many such vertical rows are created simultaneously side by side, all around the trunk of the tree, girdling it. Any one such lick site lasts for only three or four years before all of its phloem flow has been interrupted and the tree dies.

Fluid mechanics also explains why the sapsuckers end up making huge destructive lick sites on birches. When a sapsucker takes out a patch of bark on a birch tree in the summer after the leaves are on, the sap oozes down from above and eventually gets routed around the wound after the tree responds by plugging the open phloem channels. There is then still just as much phloem sap that descends down the trunk of the tree, and the bird can tap it as before by simply opening another patch of bark above where the tree had attempted to stanch the hemorrhage. Alternatively, more fluid is rerouted laterally, immediately adjacent to the original hole. The woodpecker may also take the next patch of bark out there, and so the area of little bare patches expands both vertically and laterally, gradually encircling the whole tree trunk as the woodpecker keeps a step ahead of the tree’s defense. This results in a band of dead tissue on the tree—a situation analogous to putting more and more stones into the middle of a stream to try to stop the flow, which isn’t stopped but instead, for a while, actually increases laterally. It pays the woodpecker to keep working on the same tree as the phloem flow becomes more narrowly channeled. Eventually, however, the next few taps will cut the flow off entirely, and then the tree dies. Luckily for the woodpeckers, they can simply drill into the next tree.

The isolated sap licks on birch trees are magnets for life during the summer. In 2004, when the woodpeckers established a lick station in a large birch near my cabin, I built a platform of boards in a maple tree adjacent to it, at about the same level twenty feet up. I sat there often to watch. In one perhaps typical watch (six to seven AM on 7 July 2005), when I again visited my sapsucker lick to “take its pulse” I saw five bald-faced hornets at one time. All these wasps were usually grouped at the same little square sap hole lick on one branch. Four red squirrels came for their sweets. I saw nineteen hummingbird visits (apparently, all by females or fledged young). There were eleven sapsucker visits, with up to four adults present, once, at the same time. I heard the contrasts of the resonant drone of the hornet queens versus the high whine of their smaller workers, as well as the deep hum of the ruby-throats; and also the sapsucker’s drumming, which seemed incongruous in the summer.

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Fig. 28a. Juvenile sapsucker at a typical late-summer phloem sap lick, on birch.

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Fig. 28b. Typical sapsucker xylem lick in spring, on a sugar maple. Note the tap hole at top center (current year) and the two holes at lower right (previous year).

Most of this drumming—often ten to fifteen minutes at a stretch—came from the same areas and occurred shortly after light at dawn and in the half hour before dark. No other woodpeckers were drumming then. But here they drummed—one to the west of me, one to the north, with a third joining in from the east. The sapsuckers’ drumming at this time, after the young had fledged, was curious. These birds had long since staked out territories and had already paired up in May. Were they making territorial claims on the sap station?

I am glad I got to know the sapsuckers a little better than I had known them before. Now they had become unique. It seemed that I had discovered new neighbors. And through them I had met a diversity of life at our common summer “watering place.” My summer woods are richer now than they were before.