Small birds almost overheat while feeding their young
(PHYSorg.com Biology)

For decades, researchers have thought that access to food determined the brood size of birds. Now, biologists at Lund University in Sweden have discovered a completely new explanation: the body temperature of small birds can increase by more than 4°C to exceed 45°C when they are feeding their young. Larger broods would require more work, resulting in even higher body temperatures-something the birds would probably not survive.