Black Swifts, Mt. Diablo

Ethan Monk

Today I spent the morning birding around the summit of Mt. Diablo, with good migrant numbers, and the highlight of two Black Swifts at roughly 805am. Photos are now in eBird. It should be noted that yesterday, with very similar weather, Steve Buffi reported 3 Black Swifts on Mt. Diablo, and today was a movement day for the species statewide with reports in San Diego, Monterey, etc.

I wasn't going to write anything up for today, but I think tomorrow's weather may be very similar to today's and yesterday's and might be conducive to seeing more Black Swifts up there. (And if not, hopefully replicate the decent number of migrants). This morning I parked at a turnout on Summit Rd. just before the summit (37.8802093,-121.9087847) and walked perhaps a third of a mile to this location: (37.880228, -121.908875). This morning on top of Mt. Diablo it was very clear, about 15% cloud cover or less, windless, and warm. The marine layer came up as far as Juniper Campground, and was on all sides of the mountain but North. So the summit was a "sky island," with just a couple hundred feet of mountain top above the fog. This could be seen as I was driving down Ygnacio Valley Rd. in the morning where there was no marine layer overhead, at least at 630am. Yesterday, I was in the delta and not on Mt. Diablo, but it looked like the fog had a similar extent and altitude. Today's Black Swifts first appeared in the flocks of resident White-throated Swifts, fairly low over the marine layer, before working their way up the mountain and directly overhead, and then presumably passing over the summit after spending some time circling around over me. (I must admit, I left soon after seeing these swifts since the sounds of Swainson's Thrush calling from all around me and especially from the summit made me eager to bird around before the marine layer burned off and the weekend crowds arrived. So if more Black Swifts came through after about 815am, I would not have seen them. )

I think this sky island effect might be crucial to seeing Black Swifts on Mt. Diablo. In many places overcast weather is thought of as "good" for seeing migrant Black Swifts. I would assume the same is true here--if you were a Black Swift travelling under the marine layer through the Diablo Ranges, you would eventually run into Mt. Diablo whose flanks would funnel you through the marine layer and into the open sky? The only way to prove this theory would be to test it over and over again, and see if it produces results. If you are interested in doing that, a good website I often use to check the extent of fog before birding is --today and yesterday it showed solid white fog over Diablo, but ending short of the valley floor. Of course, the big risk is that the fog goes too high, and the entire mountain is swamped in fog in which case you've wasted $10 and a morning birding.

Aside from Black Swifts, migrant numbers in the early morning were good above the fog line. The most productive site was Juniper Campground which sat pretty much right at the fog line. I turned up 14 warblers here, including 3 Hermit, and 6 Swainson's Thrush which is decent. And at the immediate summit a small flock of migrants held 6 Warbling Vireos. Strangely, I did not find a single tanager above the fog line. I am also curious if these migrant numbers are a regular thing up here in May, or are tied to the "sky island."

Anyway, if anyone gives it a shot tomorrow, good luck. The North Gate entrance often opens as early as 715am or earlier. Today it was open when I arrived at 655am.

Good birding,
Ethan Monk