During the breeding season, between May and September (2) (5), the pomarine jaeger is highly dependent on the availability of its main food source, lemmings. Lemming populations grow and shrink cyclically every three to five years, and the pomarine jaeger only successfully reproduces when lemming populations peak (6).
The pomarine jaeger catches lemmings by chasing them across the ground and, if the lemming tries to evade the bird by entering its burrow, the pomarine jaeger will tear at the burrow with its beak to reach the lemming inside (8). The pomarine jaeger is the only avian predator which will dig for the lemming in this manner (4). It will also prey on other types of rodent (3) and when these are in short supply it will eat small birds (3) (9). Outside of the breeding season the pomarine jaeger eats a smaller number of rodents and instead eats more birds, carrion, fish and insects (3), which it obtains not only by scavenging and hunting, but also by stealing from other birds (4).
The pomarine jaeger builds its nest on marshy flat land (6). The female usually lays 2 eggs which are incubated for 25 to 27 days (6). The chicks leave the nest just two days after hatching (7), but it takes a further month for their feathers to become fully developed and for them to be able to fly (6).
When protecting its breeding territory the pomarine jaeger has a very nasal call and will simultaneously flap its wings higher up and more slowly than usual. If the intruder is not deterred, the pomarine jaeger will call and start to glide with its wings held in the shape of a ‘V’ to scare off the intruder. This species may also give a short call or a call that fluctuates in pitch to warn when predators are approaching (8).