One characteristic which differentiates bats from birds is that bats

Bats are mammals, while birds belong to class Aves. Although both can fly and share certain features, the question asks for a characteristic that differentiates bats from birds - something bats have that birds do not.

The distinguishing feature is that bats possess mammary glands. Mammary glands are the defining characteristic of all mammals (class Mammalia). Female bats produce milk to suckle their young, which is something no bird does. Birds instead feed their young by regurgitation or by bringing food to the nest.

The other options describe features that both bats and birds share, so they cannot differentiate the two:

• Both bats and birds possess a four-chambered heart (two atria and two ventricles), so this is not a distinguishing feature.
• While many bat species are nocturnal (active at night), some birds are also nocturnal (e.g., owls, nightjars), so nocturnal activity is not a reliable differentiating characteristic.
• Both bats and birds are warm-blooded (endothermic), maintaining a constant body temperature regardless of the environment, so this does not differentiate them.

When comparing animal groups in examinations, focus on features that are exclusive to one group. Mammary glands are exclusive to mammals and are never found in birds.