Why did the swallow feel warm?

The statue of the Happy Prince has been weeping over the fate of the poor and needy in the town beneath his feet. He's asked the little swallow to stay behind for just one night to remove precious stones and pieces of gold from his statue and distribute them to various poor people throughout the town. Far away in the distance lives an impoverished seamstress, working her poor fingers to the bone embroidering a satin gown for the loveliest of the Queen's ladies-in-waiting. In a corner of the little room where she toils away lies her son, laid low by a terrible fever and asking for oranges to cure his raging thirst. The Prince asks the swallow to remove a ruby from the hilt of his sword and take it to the poor seamstress. The swallow does as the Prince asks, dropping off the ruby on the seamstress's table and fanning her son's fevered brow with his wings for good measure. When the swallow returns, he says that although the weather is cold, he feels quite warm. As the Prince explains, this is because the swallow has performed a good deed.