Which bird lacks distinctive "immature" plumage during its first year?

The American Kestrel is the correct choice because it is known for having plumage that does not significantly change or exhibit distinct immature coloration during its first year. Unlike some other raptor species, where juveniles exhibit different plumage that can be noticeably distinguishable from adults, American Kestrels maintain their general adult coloration even while still in their immature phase. This feature makes it easier to identify them year-round, as they lack a stark juvenile plumage that becomes developed in their later years.

In contrast, other species listed, such as the Goshawk, Red-tailed Hawk, and Merlin, show variation in plumage as they age, often requiring birdwatchers and falconers to rely on these differences for identification purposes. These distinctions are important for understanding their age and maturity, setting these species apart from the Kestrel in this regard.