On its breeding grounds, the long-tailed jaeger specialises in feeding on rodents, particularly lemmings and voles (2) (3) (5) (7) (8). The populations of its rodent prey often vary widely from year to year, peaking around every four to five years. In years of low rodent abundance, the long-tailed jaeger relies more on young birds, shrews, insects and berries (2) (3) (5) (7) (8) (9). It will also scavenge, and sometimes steals food from other birds (2) (3). The winter diet of this species is not well known, but may include fish, marine invertebrates, crustaceans and carrion (2) (3).
The long-tailed jaeger hunts rodents by hovering some distance above the ground, before pouncing on or pursuing the prey on the ground and pecking it to death (2) (3) (5). It cannot use its feet to grasp prey, instead usually shaking the rodent in the beak to break it apart (3) (5). Adult lemmings may be too large to be eaten whole, but a breeding pair will often cooperate to tear apart prey (2) (3) (5).
A highly territorial species, the long-tailed jaeger defends a large area around the nest, chasing away intruders and attacking any predators that come near (2) (3) (5) (8). The nest consists of an unlined scrape, usually on a gentle slope or low mound with a good view of the surrounding area (2) (3) (5). Nesting begins in June, after the snow has melted (2) (3) (5) (7) (8) (10). The maximum clutch size of the long-tailed jaeger is two, but its breeding behaviour is closely linked to rodent availability, and in years of lower abundance many pairs lay only one egg. In the lowest rodent years, many or even most pairs will not nest at all (2) (3) (5) (8) (10) (11).
The eggs of the long-tailed jaeger are incubated for around 23 to 25 days (2) (3) (7) (8). The chicks leave the nest one to two days after hatching, and hide some distance apart in low vegetation to avoid predators such as the Arctic fox (Vulpes lagopus) (2) (3) (5) (8). The female long-tailed jaeger undertakes most of the care of the young, while the male does most of the hunting and territory defence (3) (5). Young chicks are fed mostly on insects and berries (9) and must have rodents torn apart for them (3). However, after about a week the young can cooperate with the female to tear apart prey (5). The young long-tailed jaegers fledge after around 22 to 27 days (2) (3) (7) (8), but may not breed for the first time until they reach 3 or 4 years old (2) (3). This species is thought to live to at least eight or nine years old (3).
After breeding, the long-tailed jaeger migrates south from August to October (2) (3), although adults that have not successfully bred usually leave earlier (7). Juveniles may spend the whole of the first few years of life at sea (9). Non-breeding long-tailed jaegers may gather in small flocks to forage or roost (3).