Temperatures above normal during summer causes some stress in deer. The amount of stress is dependent on the quality of the habitat. For example, deer consume more water than any other mineral (yes, water is a mineral, a naturally occurring substance) and the amount of water deer need increases during periods of above normal temperatures. If water is limited by either quantity or quality, many of a deer’s bodily functions are limited, such as a buck transferring calcium to growing antlers, or a doe producing milk for fawns.
Deer can usually travel to find water. But if they are forced out of their home range to find water, which is rare, it requires huge amounts of energy that can’t be used for other bodily processes such as antler and milk production.
But in the end, unless there is a prolonged drought in your area, summer stress rarely has a lasting impact on deer herds. As the seasons and weather change, deer can usually deal with it just fine.