We will discuss later whether it is sap that he wishes: all that now concerns us is to note that the internal difference, whatever it is, is in favor of the tree that is dying; while the only external difference appears to be the marks left by the sapsucker. While one tree is sparingly marked by him, the other is tattooed with his punctures, placed in single rings and in belts around trunk and branches beneath every fork. It is a law of reasoning that, when every condition but one is the same and the effects are different, the one exceptional condition is the _cause_ of the difference. If these trees are alike in everything except the work of the sapsucker (the only internal difference apparently _offsetting_ his work in part), what inference do we draw as to the effect of his work?
We presume that he is killing the tree, without as yet knowing how he does it. What is his object? Good observers have stated that he draws a little sap in order to attract flies and wasps; that the sap is not drawn for its own sake, but as a bait for insects. Is this theory true?
The first objection is that it is improbable. The sapsucker is a retiring, woodland bird that would hesitate to come into a town garden a mile away from the nearest woods unless to get something he could not find in the woods. Had he wanted insects, he would have tapped a tree in the woods, or else he would have caught them in his usual flycatching fashion. There must have been something about the mountain ash tree that he craved. As it is a very rare tree in the vicinity of my home, the sapsucker's only chance to satisfy his longing was by coming to some town garden like our own.
Not only is the theory improbable, but it fails to explain the sapsucker's actions in this instance. In twenty years he was never seen to catch an insect that was attracted by the sap he drew. This does not deny that he may have caught insects now and then, but it does deny that he set the sap running for a lure. As he was never far away, and was sometimes only four and a half feet by measure from a chamber window, all that he did could be seen. He did not catch insects at his holes. He drank sap and ate bark.
Finally, the theory is not only improbable and inadequate, but in this instance it is impossible. I do not remember seeing a sapsucker in the tree in the spring; if he came in the summer, it must have been at rare intervals; but he was always there in the fall, when the leaves were dropping. At that season the insect hordes had been dispersed by the autumnal frosts, so that we know he did not come for insects.