What is wrong with that bird?

Most birds lose and replace their feathers twice each year. This process, called molt, usually happens gradually and in such a way that only the most keenly trained eyes will notice. However, when completely bald Northern Cardinals start visiting our feeders, even the novices among us know that something is amiss in the world of feather molt.

For a few weeks or longer each summer, an epidemic of baldness sweeps across the ranks of Northern Cardinals. The cause of this type of feather loss is a matter of debate among scientists, but the leading theory is that cardinals are host to parasitic mites that cause itching. Because songbirds can scratch their heads with their feet, they scratch their itchy heads so vigorously that they begin to lose feathers.

There is no direct evidence that this temporary baldness harms the affected bird, but it almost certainly can't do them any good. Baldness may affect a bird's reproductive success since plumage is an integral part of pair bonding and bald birds in harsh climates may find it significantly more difficult to deal with environmental stress.