what is the sub-species of the second bird you mentioned?
"L. d. dominicanus" and  "L. d. vetula" .
Kelp gull
The nominate "L. d. dominicanus" is the subspecies found around South America, parts of Australia (where it overlaps with the Pacific gull), and New Zealand (where it is known as the black-backed gull, the southern black-backed gull, or by its Māori name karoro). "L. d. vetula" (known as the Cape gull) is a subspecies occurring around Southern Africa.
what is a australasian gannet? A large seabird. where is it found? Steep Point in Western Australia, along the southern and eastern Australian coastline to the vicinity of Rockhampton in Queensland, as well as the North and South Islands of New Zealand, Lord Howe and Norfolk Islands. what does it eat? Forage fish and cephalopods. apart from this bird what are the other predators of the first one? Other fishes like tuna, striped bass, cod, salmon, barracuda and swordfish, as well as sharks, whales, dolphins, porpoises, seals, sea lions, and seabirds. where are they found? They are found along coastlines and also in lakes and rivers. does the original bird have a unique call? A  harsh "arrah-arrah" or "urrah-urrah". who are its predators? Southern giant petrel , kelp gull, and Pacific gull. how does the first one mate? It sexual maturity at six or seven years of age and the breeding season begins in October. what does it eat? Fish, krill, squid, offal , and waste from vessels.
wiki:24561079
Kelp gull [SEP] Introduction
The kelp gull ("Larus dominicanus"), also known as the Dominican gull, is a gull that breeds on coasts and islands through much of the Southern Hemisphere. The nominate "L. d. dominicanus" is the subspecies found around South America, parts of Australia (where it overlaps with the Pacific gull), and New Zealand (where it is known as the black-backed gull, the southern black-backed gull, or by its Māori name karoro). "L. d. vetula" (known as the Cape gull) is a subspecies occurring around Southern Africa. The specific name comes from the Dominican Order of friars, who wear black and white habits.