The great skua (Stercorarius skua), sometimes called "bonxie" in Britain, is a large seabird belonging to the skua family Stercorariidae. This article explores the dietary habits, foraging strategies, and ecological impact of this fascinating avian predator.
Appearance and Identification
The great skua is roughly the size of a herring gull, measuring 50-58 cm (20-23 in) long and having a 125-140 cm (49-55 in) wingspan. One study found that 112 males weighed an average of 1.27 kg (2 lb 13 oz) and that 125 females weighed an average of 1.41 kg (3 lb 2 oz). Adults have a streaked greyish-brown plumage with a black cap, while juveniles are a warmer brown and unstreaked below. They have a short, blunt tail and exhibit a powerful flight. A distinctive white patch at the base of their primaries is visible during flight. The great skua's call is a harsh "hah-hah-hah-hah," but quacking and croaking noises have also been heard.
Distinguishing the great skua from other North Atlantic skuas, such as the parasitic jaeger, pomarine jaeger, and long-tailed jaeger, is relatively straightforward. Its herring gull size, massive barrel chest, and white wing flashes make it easily identifiable, even from a distance. It is sometimes said to give the impression of a common buzzard. Distinguishing this skua becomes more complex when differentiating it from the closely related large southern-hemisphere skuas.
Distribution and Habitat
The great skua breeds in Iceland, Norway, the Faroe Islands, and on Scottish islands, with some individuals breeding on mainland Scotland and in the northwest of Ireland. They nest on coastal moorland and rocky islands, typically laying two spotted olive-brown eggs in grass-lined nests.
As a migrant species, the great skua winters at sea in the Atlantic Ocean and regularly reaches North American waters. It is also vagrant to Mediterranean countries.
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General Feeding Habits
The great skua is a carnivore and scavenger with a highly varied diet. They are opportunistic feeders and are also known to use fishery discards as a food source. The great skua mainly eats fish, birds, eggs, carrion, offal, rodents, rabbits, and occasionally berries. They have even been known to prey on lambs and pony foals.
Foraging Strategies
The great skua is an aerial apex predator and an aggressive kleptoparasite, known for deliberately harassing birds as large as gannets to steal their catches and often obtain fish by robbing gulls and terns. It also readily kills and eats smaller birds such as puffins.
Like most other skua species, it continues this piratical behaviour throughout the year, showing less agility and more brute force than the smaller skuas when it harasses its victims. A common technique is to fly up to a gannet in mid-air and grab it by the wing, so that it stalls and falls into the sea, where the great skua then physically attacks it until it surrenders its catch.
Using night vision gear, skuas were also observed preying on the petrels at night, a remarkable strategy for a seabird.
Dietary Specialization
In some colonies, individual great skuas exhibit dietary specialization, with some focusing on scavenging, others on predation, and others on kleptoparasitism. Some individuals regularly prey in seabird colonies, forage at sea, and alternate between both strategies.
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Diet Analysis Techniques
Knowledge of the diet of marine predators such as seabirds is fundamental to understanding the ecological cascades they may influence and the impact that environmental changes may have on them. Diet analysis of seabirds frequently relies on the identification of fish otoliths in pellets. However, it is recognised that the true dietary importance of fish with small and fragile otoliths is likely underestimated, requiring an additional method. In one study, researchers compared the identification of otoliths with that of vertebrae in pellets to gain a more complete picture of seabird diet. Atlantic herring and Atlantic mackerel were the most common fish species at all colonies when using vertebrae in contrast to being virtually absent when using otoliths. Conversely, the occurrence of Norway pout and pollock otoliths was six and eight times, respectively, higher than for vertebrae. Therefore, combining data from both otoliths and vertebrae provides a more complete profile of the fish component of seabird diet.
Breeding and Social Behavior
Great skuas are monogamous and usually form pairs that remain together for life. They nest in loose colonies, but pairs are very territorial and return to their previous nesting sites year after year.
The breeding season typically starts in May. Female Great skuas lay 2 spotted olive-brown eggs in grass-lined nests, which are incubated for 28-32 days. Chicks are precocial; they are covered with dawn and leave the nest 24-48 hours after hatching.
Interactions with Humans
Due to its size, aggressive nature, and fierce defence of its nest, the great skua has little to fear from other predators. They also show little to no fear of humans - anybody getting close to their nest will be repeatedly dive-bombed by the angry adults.
Conservation Status
According to the IUCN Red List, the total population size of the Great skua is 30,000-34,999 mature individuals. The breeding population in Europe consists of 16,300-17,200 pairs. The IUCN lists all skua species as LC or “least concern.” Due to their extensive range and large population, they do not qualify for “threatened” status. However, some species, like the brown skua, have had a population decline in recent years.
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