Apparently a juvenile dark morph. Note the relatively long tail (even without the tail streamers found on breeding adults). Also, note bold, neat barring on the undertail coverts. If you look at the close up photos, the bill is relatively thick, stubby and half black. The gonydeal angle is near the midpoint of the bill and is very inconspicous. According to Sibley, these are all characteristics of a Long-tailed Jaeger as opposed to a Pomarine or Parasitic.