34-B-2018 Parasitic Jaeger

STATUS
In 1st Round
HOW MANY OBSERVED?
1, light-phase adult
DATE SIGHTING OCCURRED
DATE REPORT PREPARED
REPORTER
Jon Curd
OTHER OBSERVERS
None
LOCALITY OF OBSERVATION
Adjacent to the Black Sands RV Resort, close to the dam on CJ Strike Reservoir, Owyhee County.
HABITAT WHERE THE BIRD WAS OBSERVED
Large body of open water.
SIGHTING DURATION, CONDITIONS, and EQUIPMENT USED
Seen off and on, over about a 2 minute period. Originally seen for just a few seconds (10?) at about 100-150 yards offshore when I spotted it from inside my vehicle with my Vortex Talon 10x42 binos chasing a gull. Once I parked, grabbed my camera, and got outside of my rig (about 30 seconds later) I was then able to relocate the bird fairly quickly, but much farther away (400? yards) with my binoculars. I immediately used my Panasonic Lumix digital camera (with 25-400 zoom lens) to try and capture images. Light winds (5-10 mph) and mostly sunny. Bird was angling into the sun when seen initially. Later, at the greater distance, it was north (mostly perpendicular) of the sun.
DID YOU TAKE NOTES?
No, not at all
DID YOU CONSULT A FIELD GUIDE OR OTHER REFERENCE WORK?
Yes, later the same day
Yes, another day after the observation
WHICH GUIDE(S) OR REFERENCE(S): DID YOU CONSULT?
National Audubon's "The Sibley Guide to Birds" (2000) minutes after the observation. The following day, multiple guides including Audubon's "Master Guide to Birding" (Volume #2 "Gulls to Dippers" - 1983), National Geographic's "Field Guide to the Birds of North America" (4th edition), National Geographic's "Field Guide to the Birds of Western North America" (5th edition) and Peterson's "Western Birds" field guide.
DESCRIPTION OF THE BIRD(s)
When I first caught sight of the bird, I was immediately struck by the long, angled wings and very deep, yet fairly fast wingbeats. The primaries showed white wing flashing with darker underwing coverts and a pale belly. The bird was noticeably smaller than the ring-billed gull it was pursuing. In the first couple of seconds of observation I knew immediately it was one of the jaeger species and moved quickly to try and get photos to assist in identification. When I relocated the bird, I spent the ~1 minute of observation trying to capture photos as it broke off the gull pursuit and flew away, fairly low over the water. I lost the bird when my camera showed the 2 second "play-back" of my latest photo and I could never relocate it again, due to the distance and backdrop/lighting involved. Several of the 7 distant photos I obtained (and included in this report) show a considerable amount of white wing-flash in the primaries when viewed both from above and below and that I was only able to view as I took the photos.
BEHAVIOR OF THE BIRD(s)
Very deep wing strokes, well below horizontal, and fairly fast paced. As mentioned earlier, this bird was in a prolonged, twisting chase/harassment of a ring-billed gull. After exiting my vehicle, I re-found the jaeger still chasing, presumedly the same gull, over a long distance (several hundred yards).
HOW AND WHEN DID YOU POSITIVELY IDENTIFY THE BIRD, AND WHAT CLINCHED THE IDENTIFICATION FOR YOU?
Not 100% clinched, but I do believe it to be a parasitic based on available evidence. The white "wing-flash" in the top-side view of the primaries is quite visible and rare on other jaeger species. See below, where 'process of elimination' of other jaeger species plays large into my identification as well.
HOW DID YOU ELIMINATE SIMILAR SPECIES? WHAT WERE THEY?
Mostly based on photos:
1) Dark underwing coverts contrasting with the lighter/whitish primaries. Long-tailed should be more uniformly dark underwing and a lighter shade (Sibley) on a light phase adult; pomarine is similar, but didn't appear (not conclusively) to show the double white-wing flash on the underneath primaries usually seen (Nat'l Geo, Birds of Western NA).
2) White wing flash visible on the upper wing is rare on other jaegers (Sibley) and looks very apparent in a couple of photos.
3) Wing shape: Parasitic has a considerably narrower wing base than pomarine; this is fairly subjective, but the narrow wing base is evident in at least one photo.
4) Very deep wingbeats, well below horizontal, typical of parasitic/pomarine, but not long-tailed (Sibley). The wing beat was fairly fast (subjective) where the pomarine is a slower paced wingbeat. The bottom end of the deep wing-stroke is very visible in one photo, as it was in my observations.
5) Prolonged chase, more identifiable behavior for parasitic; long-tailed not as aggressive and pomarine less prolonged and acrobatic (Sibley).
6) Size/shape: considerably smaller than the RBGU it was pursuing. Pomarine is larger and closer in size to a RBGU. Body also seemed too slender to be pomarine.
WHAT IS YOUR EXPERIENCE WITH THIS (AND SIMILAR) SPECIES?
Limited. Taken 6-8 pelagic trips over the past 25 years were I have encountered the 3 various jaeger species found in the ABA area including the parasitic.
DESCRIBE YOUR GENERAL BIRDING EXPERIENCE
Fairly advanced. Birding for 40+ years and almost daily the past several years.
ADDITIONAL INFORMATION (If any)
Unfortunately I never got a good look at the tail/central tail feathers or the bill when it was closer and the photos are too distant to discern much of anything.
WERE PHOTO(S), VIDEO, AND/OR AUDIO OBTAINED FROM THIS SIGHTING?
Photo
SUPPORTING IMAGES