Text 2 (Number 10-18)The Passenger Pigeon was once the most abundant bird in North America, with an estimated population of 3 to 5 billion. These birds were found in enormous flocks, so vast that they could darken the sky for hours as they passed overhead. The pigeons inhabited the deciduous forests of the eastern United States and parts of Canada, primarily feeding on acorns, chestnuts, and other forest mast. They were known for their exceptional speed and agility in flight, making them a remarkable sight in the sky.Passenger Pigeons were sexually dimorphic, with males displaying brighter and more colorful plumage than females. Males had a bluish-gray back, a rose-colored breast, and a metallic iridescence on their neck and wings, while females were generally duller in color. These pigeons had a streamlined body and a long, pointed tail, which contributed to their aerodynamic efficiency. Theirdistinctivecall was a soft cooing sound, which could be heard from miles away when the birds gathered in large numbers.The decline of the Passenger Pigeon population began in the early 19th century due to habitat destruction and overhunting. As European settlers expanded westward,theycleared vast tracts of forest, eliminating the pigeon's natural habitat. Additionally, the birds were hunted extensively for their meat, which was considered a cheap and plentiful food source. The development of railroads and telegraph systems enabled hunters to track and exploit the flocks more efficiently, leading to mass killings.By the late 1800s, the population of Passenger Pigeons had plummeted dramatically. Efforts to breed the pigeons in captivity were largely unsuccessful, and the species continued to decline. The last confirmedsightingof a wild Passenger Pigeon was in 1901, and the species was declared extinct with the death of "Martha," the last known individual, at the Cincinnati Zoo in 1914.The extinction of the Passenger Pigeon serves as a poignant reminder of the impact of human activity on wildlife. It highlights the importance of conservation efforts to protect endangered species and their habitats. Today, the story of the Passenger Pigeon is often cited in discussions about biodiversity loss and the need for sustainable environmental practices. While the Passenger Pigeon is gone forever, its legacy lives on as a symbol of the consequences of unchecked exploitation of natural resources.What does the word "they" in paragraph 3 refer to …