The Willow Flycatcher and the Least Flycatcher look similar and their ranges overlap in many regions of the United States; however, the two species do not interbreed. Based on this information, Willow Flycatchers and Least Flycatchers are distinct species.

The statement that Willow Flycatchers and Least Flycatchers are distinct species despite looking similar and sharing overlapping ranges, but not interbreeding, follows the concept of the Biological Species Concept (BSC). According to the BSC, a species is defined as a group of individuals that actually or potentially interbreed in nature. In other words, if two organisms can mate and produce viable, fertile offspring, they are considered the same species. Conversely, if they do not interbreed in the natural environment, even if they appear similar and live in the same geographic areas, they are considered separate species.

The key point here is that the capacity to interbreed and produce fertile offspring is a critical factor in defining species limits under this concept. Even though the Willow Flycatcher and the Least Flycatcher may look alike and coexist geographically (sympatric distribution), the fact that they do not interbreed suggests that there are reproductive isolating mechanisms that prevent gene flow between the two bird groups. These could be behavioral differences, such as different mating calls or courtship rituals, temporal differences in breeding times, or physiological differences that prevent successful mating or fertilization.