Summer World_ A Season of Bounty - Bernd Heinrich [67]
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.
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