Additionally, thick blubber deposits and intricate blood vessels in the flippers and legs assist in the preservation of heat. The main predator of the chinstrap penguin at sea is the leopard seal ("Hydrurga leptonyx"). Every year, the leopard seal causes the chinstraps population to decrease by about 5% to 20%. On land, the brown skua ("Stercorarius antarcticus"), south polar skua ("Stercorarius maccormicki"), and southern giant petrel ("Macronectes giganteus") are the primary predators of the penguin. These three species most often prey on eggs and young chinstrap penguins. The Antarctic fur seal is also known to occasionally kill chinstrap penguins. On land, they build circular nests from stones, and lay two eggs, which are incubated by both the male and the female for shifts around 6 days each. The chicks hatch after around 37 days, and have fluffy grey backs and white fronts. The chicks stay in the nest for 20–30 days before they go to join other chicks in a crèche. Around 50–60 days old, they moult, gaining their adult feathers and go to sea. Chinstrap penguins are generally considered to be the most aggressive and ill-tempered species of penguin. In 2004, two male chinstrap penguins named Roy and Silo in Central Park Zoo, New York City, formed a pair bond and took turns trying to "hatch" a rock, for which a keeper eventually substituted a fertile egg, and the pair subsequently hatched and raised the chick. Penguins by nature hatch eggs and are social creatures. The children's book "And Tango Makes Three" was written based on this event. The global population of the chinstrap penguins is estimated to be at least eight million. Although it is believed to be decreasing overall, its population is not severely fragmented and in many sites it is increasing or stable. The chinstrap penguin is primarily threatened by climate change. In several parts of its range, climate change decreases the abundance of krill, which likely makes reproduction less successful. For instance, a 2019 expedition to breeding grounds on Elephant Island show a fifty percent population decline in just under fifty years. Other potential threats include volcanic events and the fishing of krill by humans. Several conservation actions are taking place for this species. Multiple areas where it lives are being monitored for long periods. Conservation actions proposed for the future include more monitoring and researching of its population, range, and behavior.