The feeding habits of the birds may, from the present knowledge, be declared practically entirely beneficial. In return it seems not too much to expect that we should without complaint furnish, for a few days in the year, the drink to wash the great numbers of our insect enemies down to their destruction; and to consider these two little fellows as among the worthiest as they are among the prettiest of our warbler friends.
Prof. Maurice Brooks (1933), speaking of this warbler in West Virginia, says:
We had at that time [1909] a small commercial vineyard, and during the first week in September, when the crop was just ripening, we were surprised to find in the vineyard swarms of Cape May Warblers. We were not long in doubt as to their purpose there, for within a week they had destroyed practically every grape we had. * * * Their method was to puncture the skin of the berry at one point, extract a little juice, and move on to the next. They would systematically work over every berry in the cluster, if undisturbed, and they soon became exceedingly tame. It is no exaggeration to say that there were hundreds of the birds in the locality.
After the birds had made one puncture, swarms of bees and wasps soon finished the work of destruction. There was no way of frightening so many birds away, and we were driven to sacking our grapes in the future. The next year, 1910, they returned in numbers again, destroying practically all unsacked clusters, and completely cleaning out the vines of our neighbors, who raised just a few grapes for their home use.
These and other warblers have been seen drinking sap from the holes dug in trees by sapsuckers, but they also obtain some insects from such borings and perhaps also from the punctured grapes, which make fine insect traps. However, the damage does not seem to be universal, and occurs only where the birds are abundant, and then for only a short time. In view of his record as an insect destroyer, the laborer may be worthy of his hire.
To the insects mentioned as food for this warbler, A. H. Howell (1932) adds small crickets, flies, leaf hoppers, termites, larvae of moths, dragonflies, and daddy-long-legs.
Behavior.—Brewster (1938) writes:
It keeps invariably near the tops of the highest trees whence it occasionally darts out after passing insects. It has a habit of singing on the extreme pinnacle of some enormous fir or spruce, where it will often remain perfectly motionless for ten or fifteen minutes at a time; on such occasions the bird is extremely hard to find, and if shot is almost certain to lodge on some of the numerous spreading branches beneath. * * * In rainy or dark weather they came in numbers from the woods to feed among the thickets of low firs and spruces in the pastures. Here they spent much of their time hanging head downward at the extremity of the branches, often continuing in this position for nearly a minute at a time. They seemed to be picking minute insects from the under surface of the fir needles. They also resorted to a thicket of blossoming plum trees directly under our window, where we were always sure of finding several of them. There were numerous Hummingbirds here also, and these, the Cape Mays were continually chasing.
While watching a pair at their nest building, Dr. Merriam (1917) saw a female on the ground gathering material; she “was attacked by a Junco and after a chase the Junco actually caught and held her. At this commotion the male Cape May flew down and lit close by but took no active part in the argument. The Junco was apparently victor for after one more flight to her nest the female Cape May was not again seen to trespass on the Junco’s territory or do any more nest building that morning.” However, in his notes from West Virginia, Dr. J. J. Murray says that “this warbler is more active and restless in its feeding than any of our warblers, except possibly the myrtle; and it is also noisier and more aggressive in its attitude toward other warblers which seek to share its feeding places.” Harlow also says that “the male Cape May is the tiger of the north woods in defending his territory. He attacks all birds that come close to the nest, up to the size of the olive-backed thrush, and is absolutely fearless.”