Passage raptors can appear larger than adult birds primarily due to their fresher plumage and the vigorous physical condition that often accompanies their migratory behavior. During migration, these young birds are typically in excellent health and may have more robust body mass compared to adults, who may have experienced wear and tear from previous breeding seasons. This condition can create an optical illusion where they seem larger, especially when compared to adult birds that have dulled plumage and potentially reduced body weight due to energy expenditure during breeding.
Additionally, young passage raptors are often observed in groups during migration, which can also enhance the perception of size as they are seen alongside adults. Their readiness to engage in hunting and flight can further amplify the perception of size when observed in a natural setting.
The other options do not accurately represent the characteristics of passage raptors. They do not only appear larger at specific times of the year or when in flight, and there are consistent visual differences throughout their migratory periods compared to adult birds.