Here's what Birds of North America has to say about display flights -
does this description fit what you saw?

[Birds of North America online, Northern Harrier section]
Males advertise territory occupancy and court females by performing
Sky-Dancing Display. Each aerial display comprises a sequence of as
many as 74 deep, U-shaped undulations covering a distance of up to 1
km at a height of 10?300 m; most bouts about 25 Us at 20 m above
ground (Simmons 1988a; see Breckenridge 1935 for full description).
Females sky-dance, but less frequently than males (Simmons et al.
1987, RBM). Sometimes accompanied by chattering vocalizations at the
zenith of undulations (Saunders 1913, RBM). Frequently ends with male
disappearing into a potential nest site. Female attracted by display
typically follows and displaces male from the site, suggesting
display plays a role in nest-site selection (Simmons 1991a). Adult
males display more often and more intensely in food-rich than in
food-poor years (Simmons 1988a); precipitation reduces frequency
(Follen 1986, RBM). The most vigorously displaying males attract the
largest harems (Simmons 1988a). Males will sky-dance over suitable
nesting areas while on migration, so sky-dancing may not always
reflect ownership of a breeding territory (Hamerstrom 1969).
[/Birds of North America online, Northern Harrier section]
In a later section, they also mention these display flights occurring
in late-March to early April, so the timings right too.

Matt Bartels
Seattle, WA
>Tweets,
>Monday on Vantine Rd. near Tenino I observed a female harrier doing
>aerial acrobatics I've never seen before. Twice in succession it
>dived while twisting/rotating, then regained its elevation in kind
>of a loopty-loop, and was even flying upside down. My view of what
>was happening at the bottom of the dive toward the treetops was
>largely obstructed, as this occurred about 40 yards from me looking
>upward from the base of a hillside/ridge with ~10-year-old planted
>fir trees. The bird was also calling/crying with several detached
>distress-sounding notes, which I don't recall hearing before either.
>I'm wondering whether it was stooping on prey in the thick stand
>(I've seen harriers work this wooded ridge twice before, but it's
>unusual), chasing off a predator, or engaging in a mating display.
>Shortly a male harrier appeared flying off over the short treetops
>(I could not see from where, other than it was very nearby), and the
>two flew off together in normal flight.
>I had been tipped off that something was up. A half-mile or so
>previous about 10 jays got all worked up and were headed somewhere
>toward this neighborhood to investigate. At my next pulloff (this
>location) I saw a few feathers floating in the air. And a number of
>other songbirds were very agitated, especially a lone Hairy
>Woodpecker at nearly ground level giving distress calls. My first
>thought was that a pygmy owl, a regular here, took out the Hairy's
>mate. (I ended up seeing in the vicinity 3 pair each of Downys and
>sapsuckers in the vicinity, plus numerous flickers--all were very
>active). Any thoughts on the harrier's behavior?
>Also had my first singing Yellow-rump Monday.
>Good birding!
>Paul Hicks
>Tenino, s Thurston Co
>phicks AT accessgrace.org
>
>
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>Tweeters at u.washington.edu
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