Final answer:
Deer increase their activity and roam outside their territory due to seasonal and behavioral changes, often during autumn or spring. Migration occurs in response to forage availability and environmental conditions like the accumulation of body fat for survival.
Step-by-step explanation:
Deer are likely to be more active and roam outside of their territory during periods of seasonal changes and corresponding behavioral changes. These changes are often seen with the approach of autumn when they may be seeking mates during the rutting season, or during spring when forage is more plentiful and fawns are being born. Additionally, changes in seasonal availability of food resources can lead to migration, such as with elk moving from high mountain meadows to lower valleys in winter.
This migration and increased activity also coincide with the need to accumulate energy reserves, such as body fat, to ensure survival during harsher seasons, as studied in elk populations in Yellowstone National Park.