In the Lassen Peak region, Grinnell, Dixon, and Linsdale (1930) report four nests in willows, one in a wax-berry (Symphoricarpos), one in a snowbush, and one fastened between stems of rose and willow at the edge of a clump of rose.
Eggs.—Three or four eggs seem to constitute the commonest set for the California yellow warbler. These are hardly distinguishable from those of the eastern bird, though they average, perhaps, more heavily marked. The measurements of 40 eggs average 16.6 by 12.4 millimeters; the eggs showing the four extremes measure 18.3 by 13.5, 17.8 by 15.0, 14.7 by 12.2, and 16.3 by 11.4 (Harris).
Food.—Prof. F. E. L. Beal (1907) analyzed the contents of 98 stomachs of California yellow warblers, and found that the animal matter amounts to 97 percent of the food, consisting wholly of insects and a few spiders.
The largest item is Hymenoptera, which amounts to over 30 percent, almost half of which are ants. The remainder are small bees and wasps, some of which are probably parasitic species, though none were positively identified. * * * Caterpillars, with a few moths, aggregate over 18 percent.
Beetles form nearly 16 percent of the diet, and embrace about a dozen families, of which the only useful one is that of the ladybirds (Coccinellidae), which are eaten to a small extent. The great bulk of the beetle food consists of small leaf-beetles (Chrysomelidae), with some weevils and several others. One stomach contained the remains of 52 specimens of Notoxus alamedae, a small beetle living on trees. Bugs (Hemiptera) constitute over 19 percent of the food, and are eaten regularly every month. Most of them consist of leaf-hoppers (Jassidae) and other active forms, but the black olive scale appeared in a number of stomachs. Plant-lice were not positively identified, but some stomachs contained a pasty mass, which was probably made up of these insects in an advanced stage of digestion.
Flies seem to be acceptable to the summer warbler; they are eaten to the extent of nearly 9 percent. Some of them are of the family of the house fly, others are long-legged tipulids, but the greater number were the smaller species commonly known as gnats. A few small soft-bodied Orthoptera (tree-crickets), a dragon-fly, and a few remains not identified, in all about 5 percent, made up the rest of the animal food.
Only about 21⁄2 percent of the food was vegetable matter, made up mainly of fruit pulp in a single stomach, one or two seeds and rubbish.
Rathbun (MS.) says that this warbler “shows some partiality for feeding on aphids, for we have many times watched it in an orchard carefully scanning the leaves on a tree for this insect.”
All other phases of the life histories of this and the following two subspecies do not seem to differ materially from those of the eastern yellow warbler and need not be repeated here.
Fall.—According to Rathbun’s notes, all the resident, breeding yellow warblers have departed from Washington “by the latter part of August, and in some seasons we have not heard the bird after the twenty-fifth; it is one of the few species that sing more or less during all of its sojourn here, and its song in late summer is almost as good as on its arrival in the spring. A break in the movement south of this species seems to occur about August 20 to 25. Then, early in September, the notes of the yellow warbler begin to be heard again. We have the idea that these may be of the Alaska yellow warbler.”