Two species of closely related swallows live on the same continent. The black swallow lives in coniferous forests, and the yellow swallow lives in deciduous forests. On an island where the black swallow has never been introduced, the yellow swallow lives in both coniferous and deciduous forests. Explain this difference in observed yellow swallow habitats in terms of niches and interspecific competition.
The two occupy different niches defined by tree type. Due to lack of competition for coniferous habitat on the island, the yellow swallow can occupy this niche as well as its original one. But if black swallows are ever introduced to that island, the yellow swallows might well be displaced from coniferous areas.