Parasitic jaegers are seabirds in the skua family Stercorariidae. There are three species of parasitic jaeger: the Arctic jaeger, the long-tailed jaeger, and the pomarine jaeger. Parasitic jaegers are medium-sized predatory seabirds that get their name from their habit of harassing other birds to force them to release their food, which the jaeger then steals – a practice known as kleptoparasitism.
Parasitic jaegers vary in size between the three species. Overall, they range in length from 38-52 cm (15-20 inches) and in wingspan from 95-125 cm (37-49 inches). Their size allows them to be agile and fast fliers over the open ocean, capable of catching bird and insect prey on the wing. However, there is variation in body size that relates to each species’ breeding habitat and strategy.
Size Differences Between Parasitic Jaeger Species
The three parasitic jaeger species show distinct size differences:
Arctic Jaeger
– Length: 38-46 cm
– Wingspan: 98-110 cm
– Weight: 285-520 g
The Arctic jaeger is the smallest of the parasitic jaegers. It has a compact, rounded body shape suited for making quick turns and accelerations in flight. This aids its aerial hunting of small birds and insects. The Arctic jaeger’s small size allows it greater maneuverability than the larger jaegers when harassing other birds for food over the open ocean.
Long-tailed Jaeger
– Length: 43-52 cm
– Wingspan: 115-125 cm
– Weight: 330-660 g
The long-tailed jaeger is intermediate in size between the Arctic jaeger and the pomarine jaeger. It has a medium-sized, streamlined body shape ideal for sustained cruising flight over the ocean. The long tail streamers of adult birds give them greater agility for catching prey. The long-tailed jaeger’s size provides a balance of speed, stamina, and agility needed for hunting other seabirds.
Pomarine Jaeger
– Length: 48-52 cm
– Wingspan: 112-122 cm
– Weight: 500-880 g
The pomarine jaeger is the largest parasitic jaeger species. It has a robust body shape that provides power and strength used to prey on large seabirds. The pomarine jaeger’s large size and hooked bill allow it to harass and kill seabirds as big as himself. This comes in handy on the wintering grounds where food is scarce. The pomarine jaeger’s greater size comes at a tradeoff of maneuverability compared to the smaller jaegers.
Size and Breeding Strategies
Differences in body size between the parasitic jaeger species correlates with differences in their breeding habitats and strategies:
Arctic Jaeger
– Small size allows greater agility for aerial displays
– Can make quicker movements to defend small breeding territory
– More maneuverable when harassing birds for food over open ocean
Long-tailed Jaeger
– Medium size balances speed and stamina for prolonged hunting
– Can patrol and defend larger breeding territory
– Aerobatic displays less agile than Arctic jaeger
Pomarine Jaeger
– Large size provides power to wrestle large prey
– Can displace other birds from nesting sites
– Less aerial maneuverability than smaller jaegers
– Relies more on brute strength than aerial agility
Overall, the smaller Arctic jaeger is adapted for hunting agility and defending small breeding territories. The large pomarine jaeger has greater strength for taking large prey and competing for nest sites. The long-tailed jaeger is intermediate – sacrificing some agility and strength to balance both.
Relative Size Differences
The differences in average size measurements of the three parasitic jaeger species are summarized below:
Species | Length (cm) | Wingspan (cm) | Weight (g) |
---|---|---|---|
Arctic Jaeger | 38-46 | 98-110 | 285-520 |
Long-tailed Jaeger | 43-52 | 115-125 | 330-660 |
Pomarine Jaeger | 48-52 | 112-122 | 500-880 |
This table summarizes the size ranges. In general:
– The Arctic jaeger is the smallest in length, wingspan, and weight.
– The pomarine jaeger is the largest in all size measurements.
– The long-tailed jaeger is intermediate in size between the other two species.
The large size variation reflects differences in the species’ breeding ecology and hunting strategies.
Male vs Female Size
There is also sexual dimorphism in size between male and female parasitic jaegers. Females tend to be slightly larger and heavier than males of the same species.
For example, in the pomarine jaeger the average measurements are:
Male
– Length: 48 cm
– Wingspan: 117 cm
– Weight: 660 g
Female
– Length: 51 cm
– Wingspan: 122 cm
– Weight: 880 g
The female pomarine jaeger averages around 3 cm longer, with a 5 cm greater wingspan and over 200 g greater mass than the males of the species.
This size dimorphism is likely related to differences in breeding roles. Females develop larger bodies to produce and incubate eggs. Males are slightly smaller and more agile for territorial displays and defending the nest site.
Despite these average differences, there is considerable overlap in size between individual males and females across the parasitic jaeger species.
Changes in Size With Age
Parasitic jaegers exhibit changes in body size as they mature from juveniles to adults:
Juveniles
– Smaller in all body measurements
– Underdeveloped flying skills and poor at hunting
– Reach adult size after 1-2 years
Adults
– Full adult size reached for breeding
– Long tail streamers in long-tailed jaeger
– Experienced, accomplished hunters and fliers
Juveniles hatch smaller than adults. They take time to develop their flying muscles and coordination. Around the time they reach sexual maturity, they attain full adult size – although some may still lack the full tail streamers.
Adults maintain their large size and flying prowess for the rest of their breeding lifespan. Size slowly diminishes as birds reach old age.
Changes in Size With Season
Parasitic jaegers undergo some seasonal changes in body mass tied to breeding, migration, and food availability:
Breeding (Spring/Summer)
– Higher energy demands can reduce body mass
– Males lose mass through aerial displays
– Females lose mass by producing eggs
Migration (Fall/Spring)
– High activity flying can burn fat stores
– Some weight loss flying over long distances
Wintering (Late Fall/Winter)
– Gain mass rapidly when food is abundant
– Provides energy reserves during food scarcity
– Better insulates birds in cold environments
Parasitic jaegers tend to be lighter during breeding season as they invest energy into reproduction over personal maintenance. They are often heaviest during wintering when building fat stores for insulation and energy. However, these seasonal mass changes are relatively small compared to differences between species, sexes, and age classes.
Size Compared to Related Bird Groups
Compared to other related groups of seabirds, parasitic jaegers are medium in body size:
Alcids (auks, murres)
– Mostly smaller than jaegers
– Examples: dovekies, murrelets
Larids (gulls)
– Variable in size – some smaller, some larger
– Examples: kittiwakes, herring gulls, great black-backed gulls
Gaviiformes (loons)
– Larger and bulkier overall
– Built for diving rather than agile flight
Procellariiformes (petrels, shearwaters)
– Often smaller than jaegers
– More adapted for gliding flight
The compact, medium size of parasitic jaegers suits their roles as aerial predators and kleptoparasites. Their size balances speed, agility, and power for hunting other seabirds on the wing.
Conclusion
In summary, parasitic jaegers occupy a medium body size compared to other seabird groups. The three jaeger species show differences in size that correlate with adaptations to their breeding ecology and hunting strategies. The Arctic jaeger is the smallest and most agile species, while the pomarine jaeger is the largest and most powerful. Females are slightly bigger than males in each species. Size increases from juvenile to adult birds. And body mass fluctuates seasonally with breeding, migration, and food availability pressures. Their specialized body size allows parasitic jaegers to thrive as aerial predators and kleptoparasites in the challenging marine environment.