A.I. Explanation:

Option 1: "their habitat requires"

Explanation: This option does not fit the context of the sentence. The text discusses the unique flying abilities of hummingbirds and their coloration, not their habitat needs. Therefore, this option introduces information that is not present in the text.

Option 2: "females especially"

Explanation: This option correctly completes the sentence by referring to the text's mention of 'the beautifully coloured male birds, more than (females especially).' It highlights the contrast between the males and females, aligning with the main idea of the text regarding the coloration of hummingbirds.

Option 3: "the younger birds"

Explanation: This option is incorrect as it introduces a new subject not mentioned in the text. The text focuses on the coloration of male hummingbirds compared to females, and does not discuss age or younger birds.

Option 4: "when they are in flight"

Explanation: This option is also incorrect because it does not relate to the context of the sentence. The text does not discuss the timing of the hummingbirds' actions in relation to their flight, making this option irrelevant.