Keeping warm
There are pros and cons to being warm-blooded: a body can keep active, even in places that are cold, but the smallest birds and mammals pay a hefty price in fuel consumption. The tiniest bodies lose the biggest proportion of body heat and thus chill more easily in the cold. As a result, hummingbirds and shrews which are smaller than the biggest insects and spiders need a constant supply of high-energy food just to survive.
