Final answer:
Heather birds differ from sparrows in their ability to fly against wild blue horizons and represent a sense of freedom and adventure.
Step-by-step explanation:
The heather birds mentioned at the end of the story differ from sparrows in several ways. First, sparrows are commonly found in urban environments and are smaller in size, while heather birds are known for their ability to fly against wild blue horizons. Additionally, the narrator mentions sparrows twice earlier in the story, suggesting that they are a more familiar and ordinary bird. On the other hand, the heather birds represent a sense of freedom and adventure that resonates with Millicent.