Date: 6/23/20 12:58 pm From: Chris Rurik <chrisrurik...> Subject: [Tweeters] Black swift phenomenon
[I tried to send this last week. Better late than never.]
I too spent an hour [on June 15] transfixed by the black swift phenomenon. From my perch near NE Seattle's Magnuson Park I could see ~50 concentrated over Promontory Point, and small groups were almost always breaking away to arc over the surrounding neighborhoods. At times they were within 50-100 feet of my head. Once or twice I heard the whooshing of their wings. Never did I expect to have such a wonderful opportunity to study black swifts.
Pondering what might have caused the event has raised a number of questions for me:
What were they eating? The general black swift birding knowledge around Puget Sound, in my experience, goes like this: lowland black swift sightings are the result of weather conditions. Namely low dark cloud cover that makes foraging impossible in the mountains. Is there more nuance to the expected weather conditions? For example, last night was incredibly calm in terms of wind.
In the literature summarized by Birds of the World, far more attention is paid to prey conditions than climatic conditions (though weather is mentioned as playing a role in black swift foraging grounds). A number of studies from different parts of their range show that black swifts opportunistically seek out areas where flying insects are swarming or at least abundant. Flying ants are mentioned a number of times as particularly favored. Often flying swarms of insects are ephemeral mating events lasting a few days at most and sometimes just an evening. Though black swifts will take a wide variety of insect types (67 families documented), gut contents of individual black swifts often show many prey items of a single species, reinforcing the idea that they seek out areas where a single insect is having a mass flight event. What could they have been eating last night? Was there an insect flight event, or were they eating a variety of species? In my experience you know if an ant flight is on, and I have not seen any flying ants on the ground yesterday or today. There were a few small clouds of some tiny insect near my balcony, but I was unable to sample them. When the black swifts were flying low around me I failed to see any flying insect activity around them. But they were moving fast and light conditions were not great. Or perhaps the prey was on the smaller side.
Anyone have any idea what they were eating? Perhaps if we learn what attracted them here (if it is not merely weather) we can be on the lookout in the future. Stomach contents of black swifts taken in Washington have included flying ants, wasps, caddisflies, crane flies, other flies, beetles, termites, aphids, leafhoppers, treehoppers, moths, and even spiders (presumably ballooning spiders?).
Did others notice them concentrating over certain spots like the "sky island" of Promontory Point, where a certain type of prey may have been originating?
Others reported common nighthawks in Seattle. Were these typical migrants that were seen because more observers than usual were looking up? Or were they too attracted by whatever attracted the black swifts?
In response to Larry's question, I did see one Vaux's swift among the black swifts. From one to five Vaux's swifts has been a near guarantee at Magnuson Park for at least the last month. I did not see any interaction between black and Vaux's swift.
Those black swifts sure electrified the sky. The last stragglers departed by 9:15. On a brief walk the following morning I did not see any swift action, though it sounds like there were some down by the arboretum. The sky felt empty without them.