What is a brood patch on a bird?

A puff of  air on the belly of a Barn Swallow

reveals an incubation patch.

As the days lengthen, one physical change birds of nearly all species go through is the loss of feathers on their belly early in the breeding season. Nesting birds develop bare patches called incubation or brood patches just before the first egg is laid. When the feathers are fluffed, you won’t notice a change on healthy birds.

Development of these brood patches is prompted by rising levels of hormones. They form in whichever sex cares for the eggs and young, usually females but often males as well. The skin swells and the blood vessels feeding the skin expand.

The bare patch is revealed when a bird settles in to the nest, spreads apart their contour feathers that remain over the patch, so that bare skin rests directly on the eggs. This contact transfers body heat to incubate the eggs and chicks.

Later the lost feathers are replaced in the complete molt following the breeding season. 

Source and photo credit to: Avian Reproduction: Clutch Size, Incubation, & Hatching - http://people.eku.edu/ritchisong/avianreproduction2.html

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