OLIVE-SIDED FLYCATCHER. Nuttallornis borealis (Swainson)

Field characters.—Smaller than Robin but large for a flycatcher. Head big and tail short as compared with other flycatchers. Middle of lower surface of body from chin backward yellowish white; plumage elsewhere solid olive brown save for patch of white on flank. Perches at tops of tall coniferous trees whence it flies out in typical flycatcher manner after passing insects. Voice: Song a loud clear far-carrying wher, whee', whew; call notes a softer puck, twice or thrice repeated, most often heard in evening.

Occurrence.—Moderately common summer visitant to Transition and Canadian zones on west slope of Sierra Nevada. Passes through lower zones on both slopes during at least the spring migration. In Yosemite Valley, at least in early summer, but not plentiful there. Arrives in nesting range from middle of May to early June and departs about the end of August. Solitary or in pairs.

The Olive-sided Flycatcher is the patrician among the flycatchers, as it arrives late in the season and departs early, and while here maintains itself in seclusion from most other birds by keeping to the tops of the tallest trees. It spends the winter months in Central America or northern South America and so is a far traveler during its absence from our mountains. During late May and early June birds of this species are to be heard or seen, from time to time, in the foothill country where they pause to rest and feed before resuming the journey to the forests they quitted the previous summer.

At Blacks Creek west of Coulterville, on May 11 and 12, 1919, Olive-sided Flycatchers were moving past our camp on their return to the mountains. One was seen to perch momentarily on a convenient power wire over a greasewood-covered hillside before going on in a northeasterly direction toward the adjacent mountains; another heard calling from a solitary digger pine in the early morning was gone when looked for a little later. At Pleasant Valley, on May 23, 1915, individuals were observed in similar leisurely movement, going toward the cooler pine forests not many miles distant. At Mono Lake Post Office the species first appeared in migration in 1916 on May 22. The latest fall record is for September 1, 1915, when two birds were seen in Jeffrey pines at the head of Sunrise Creek, near Clouds Rest.

The Olive-sided Flycatcher is one of the earliest birds to call in the morning and one of the last to be heard in the evening. This is probably due in some degree to its choice of surroundings, for in the tree tops it is apprized of the coming of dawn long before that news reaches the earthward dwelling species, and in the same places it enjoys the lingering daylight for some time after the glades and thickets below are lost in the shadows of the evening. At Chinquapin on June 17, 1915, where these birds were already located for the summer the clear three-syllabled song, wher, whee', whew, or oh see' view, was heard at the faintest trace of dawn; and again at the same place, on May 19, 1919, a pair over our camp closed the day with their softer puck, puck, puck, continuing until after seven o'clock in the evening. Similar observations were recorded elsewhere.

At times the clear simple notes of this flycatcher are replaced momentarily by a kingbird-like bickering and the two birds of a pair will flutter far aloft around the nest site in the manner common to their noisy low-zone relatives. When feeding young they occasionally indulge in this sort of behavior, but for the most part they are of quiet demeanor. The observer may gaze into the tree tops whence the notes are proceeding for minutes at a time before detecting a movement which will reveal the source.

On June 3, 1915, an Olive-sided Flycatcher was collected from a tree at the margin of the pine forest above Coulterville. Dissection showed that the bird was not yet nesting and was likely still in migration. The stomach of this bird contained beetles one or more of which were of species which usually dwell on the ground (family Carabidae) and which do not often ascend trees; presumably they were taken in one of the rare flights of these insects. This species of flycatcher is known to nest regularly a few miles farther to the east, at Dudley on Smith Creek, where young just out of the nest were seen July 19, 1920.