Antarctic shag

The current population of the Antarctic shag is stable,[3] with a estimated 20,000 individuals worldwide.[4] Currently there is no particular threat that could hurt the community or population size.[3]

It is very hard to estimate the population of species in isolated areas, such as the Antarctic. Still, recent studies using Unmanned Aerial Vehicles (UAV) have allowed scientists to estimate Antarctic shag populations by taking images of colonies. Since this data is based on images, more can also be learned about topography, habitat selection and nesting. In the future, this might allow scientists to more precisely estimate population size.[10]

In 2018, a study found that 3.5% of the global Antarctic shag population breeds in Ryder Bay.[11] The study also calls for greater protections of the area.

Antarctic shags are monogamous and only mate with one partner each nesting season. Still, partners may change between seasons. Males attract breeding partners with a greeting display.[2]

Typically, colonies breed on low rocky cliffs near the water.[2] Sometimes, breeding colonies are shared with other bird species such as the rock shag, southern rockhopper penguin, and black-browed albatrosses.[4] This species usually forms smaller colonies of 20-40 pairs, but larger colonies of up to 800 pairs have been observed.[3]

Both genders build a nest from feathers, seaweed, and ocean debris. Materials are then connected with excreta,[2] which is waste material, such as feces and urine.[12] Shag mating pairs often steal nesting material from other couples.[4] The final nest shape looks like a cone with the tip cut off, similar to a volcano.[4] Nests are sometimes reused between years as many individuals return to the same breeding site.[3]

Antarctic shags lay their eggs between October and December. The female lays 2 or 3 eggs on average;[2] but up to 5 eggs have been observed.[4] Both parents help incubate the eggs for 28–31 days.[2] Chicks hatch without a protective down making them vulnerable to the Antarctic conditions. Due to this lack of protection, parents must keep their chicks warm for the first few weeks.[4] The naked chicks are fed by their parents for about 3 weeks with the male partner providing most of the food. After about 3 weeks, the chicks start to fledge.[2] By age 4, the offspring will have reached sexual maturity.[3] The Antarctic shag has an estimated life expectancy of 15–20 years in the wild.[4]

Breeding success can be directly affected by food availability[3] and diving conditions.[13]

The most common way to study the Antarctic stag's diet is by analyzing its pellets.[14] Their pellets are often composed of bones, fur and feathers.

Antarctic shags usually forage alone or in small groups.[4][2] Most of the Antarctic shag's diet is composed of fish,[13] but can also include crustaceans, octopuses, snails, worms, slugs and other invertebrates.[4] The fish diet is mostly composed of humphead notothen (Gobionotothen gibberifrons), gaudy notothen (Lepidonotothen nudifrons), bullhead notothen (Notothenia coriiceps), and Antarctic spiny plunderfish (Harpagifer antarcticus).[13] Typically, the females consume more invertebrates while the males consume more fish.[15] This difference in diet is most likely due to the difference in size of each gender.[14]

Antarctic shags are rarely prey for other species. There have been a few documented cases, including leopard seals and brown skuas killing Antarctic shags. However, other bird species prey on their eggs and chicks as well.[15]

Unlike other birds, the Antarctic shag consumes demersal fish.[15] In fact, this species can reach extreme depths when diving for their food.[2] Since demersal fish are found on the ocean floor, these fish feed on other benthic species.[16] Thus, the Antarctic shag links the benthic and pelagic zones of the food web. Despite linking these two areas of the ocean, the Antarctic shag has a limited population. Therefore, the amount of demersal species consumed doesn't impact the entire ecosystem.[15]

On average, the Antarctic shag dives 25 meters to feed on fish.[2] Their maximum dive depth is around 60 meters.[4] The number of dives and depth of each dive is related to foraging conditions. They most likely ingest smaller fish and invertebrates while underwater. In contrast, larger fish, 15 cm or longer, are brought to land to eat.[13]

Shags can't predict how much time is required to dive and capture prey. Once underwater, the bird looks for prey, and reacts based on the situation. If a fish is found but the shag doesn't have enough oxygen, it will come back up to the surface, take in the largest amount of air possible, and dive again to capture the prey. This shows that the Antarctic shag changes its diving strategy based on the situation.[13]

Many birds can't fly with wet feathers and diving in the water can cause feathers to become waterlogged (fully saturated). Often, you see waterbirds standing with their wings spread to dry off their feathers after diving.[17] Antarctic shags are unique in this respect and due to their dense inner plumage, this species doesn't need to dry off via wing-spreading like other diving bird species.[2] These dense feathers close to the skin also prevent icy waters from hitting the skin. Additionally, birds having to spread their wings to dry could have further reduced their body temperature in an already extremely harsh climate. Therefore, both the cold water and cold air could have caused this species to develop over time.[18]

Shags are known to be quiet but often vocalize at breeding sites or when vulnerable.[19] When threatened, the male makes a "aaark" call while the female will make a hissing call.[2] In contrast, during breeding, males makes a "honk" call.[2]