Bird of the Month: Parasitic Jaeger

By Andy McCormick

Stercorarius parasiticus

Length 16.5”

Wingspan 46”

Weight 1 lb (470 g)

AOU Alpha Code PAJA

A slim gull-like bird that flies powerfully like a falcon, the Parasitic Jaeger is the jaeger most commonly seen from land during migration.

KLEPTOPARASITISM IS THEIR GAME

Stealing food from other birds is the primary foraging pattern for the Parasitic Jaeger. This behavior called kleptoparasitism is also practiced by other birds, most notably the Bald Eagle. Jaegers often team up to harass a tern or alcid into dropping whatever meal it might be carrying. Its powerful and angular flight is falcon-like and one clue to identifying them from a distance is the vertical attack flights it can make when harassing another bird (Dunne).

Parasitic Jaegers, as all jaegers, are best seen from a boat. They spend most of the year out over the ocean, but Parasitic Jaegers come closer to land during spring and fall migration when they follow gulls and terns into bays and inlets. The Parasitic is the middle jaeger – smaller than the bulky Pomarine Jaeger and larger than the tern-like Long-tailed Jaeger.

BOTH LIGHT AND DARK MORPHS

Even in good viewing conditions identification of jaegers is difficult. There are size overlaps among the three jaegers and females are generally larger than males. Polymorphism adds to the complexity of accurately identifying jaegers. The Parasitic has both light and dark morphs and in marine waters off Washington the light morph predominates. The light morph Parasitic Jaeger has a black cap with yellow on the side of the face, and a dark breast band. The Long-tailed does not have a breast band, and the Pomarine is much bulkier than the Parasitic (Alderfer).

All three jaegers and the related skuas share the genus Stercorarius, which in Latin refers to dung. The food disgorged by other birds when pursued by jaegers and skuas was once thought to be excrement. The species epithet, parasiticus, is also Latin, and refers to the jaegers’ penchant for stealing food from other birds. Jaeger is derived from the German jaeger, which refers to hunting (Holloway).

STATUS AND DISTRIBUTION

Parasitic Jaegers are Holarctic breeders in Alaska, Canada, Greenland, Iceland, Scotland, and Norway. They migrate to the southern oceans in winter. Spring migration occurs from April to June and these jaegers have been observed on 80% of offshore trips in May. From August to November they will be in offshore Washington waters, and can be seen in 74% of offshore trips in August and September (Wiley and Lee). In May 2019 (dates 21-29) Parasitic Jaegers were seen frequently in Puget Sound between Alki Point and Bainbridge Island, including a high count of eight individuals on May 25 off Discovery Park (eBird data).

These jaegers arrive on the breeding range in late May in North America and early June in Eurasia, where they locate a slight rise in the wet tundra for the nest site and perching. These slight rises are created over time by repeated nesting and are sometimes referred to as “skua hummocks” (Wiley and Lee). Videos of Parasitic Jaegers showing the skua hummocks, flight pattern, and tail length can be seen at the Macaulay Library

TUNDRA NESTER

Parasitic Jaegers typically take three to seven years to mature for breeding. Light morph birds most frequently breed at four years of age with dark morph birds waiting one or two more years. Two eggs are usual in the clutch, but many new pairs will produce only one chick (Wiley and Lee).

The nest is a shallow depression in the earth, moss, or lichen formed by the female. Usually one to three olive-colored eggs are deposited and incubated by both parents. After hatching both parents share hunting and feeding and sometimes leave the nest together for long periods. The young can fly after three weeks but stay with the parents a few weeks longer (Kaufman). All birds usually leave the breeding grounds by late August to mid-September.

References available upon request from amccormick@eastsideaudubon.org.