Answer:

We have here a motley-colored, oddly shaped raptor in flight. Look for structural marks and key plumage features - though beware that hawks are varied and often possess several color morphs, as well as different plumages through their ages.

Despite the lack of anything to compare size to, the big in question appears to be a medium-large hawk. By the relatively shorter tail and shape of the wings, it is likely in the genus Buteo. The mottled dark brown on the belly and wing patterns are confusing, but possibly indicate an immature light morph.

LA County has four regular and four rare Buteo species: Red-tailed, Red-shouldered, Swainson’s, Ferruginous, and Zone-tailed, Broad-winged, Rough-legged, Harris’s, respectively.

We can immediately eliminate Red-shouldered by the color scheme and structure: young Red-shoulders have broader black and white barring on the tail, and reddish underwing coverts. Adults are distinctly rusty all over the underside of the body, and both adults and immatures are shorter and thicker winged than the bird in question.

Ferruginous seems a better match in wing shape, but all ages should possess an entirely white undertail — the bird here has indistinct, thin dark bars.

Red-taileds are notoriously variable and are difficult to eliminate here. Red-taileds have very broad and stout wings, which does not seem to match the bird here. However, the mystery hawk appears to be gliding and has its wings slightly swept back, so we’ll keep Red-tail as an option.

Swainson’s is another good choice. They have very long, narrow wings that taper slightly towards the tips. Another good mark for young Swainson’s is that the undertail coverts are always pale, including in dark and intermediate morphs.

The rarer buteos usually appear in LA only during the fall months, but we’ll check them to be sure. Adult and immature zone-tailed hawks have all-dark bodies, so this is not one of them. Broad-winged is overall much more compact-bodied and short winged, with more distinct markings underneath. Rough-legged is very rare and has dark patches on the “wrists” of the underwing, which this bird lacks, so we are left with Red-tailed and Swainson’s.

Having slender wings, clean white undertail coverts, and lacking dark patagial marks, this bird is a Swainson’s Hawk. Their migration lasts from March through late May in LA County, so a May 12 date is on the late side but not unusual.

Congratulations to Steven, Alex, and Riley for figring out Swainson’s Hawk!