Some sort of disease in birds

Re: Bird Baldness

Although disease can never be ruled out, given that the birds in your photos look fairly healthy I would suspect that the baldness is due to squabbling/ aggression/ over preening between nestlings whilst the birds were still in the nest.  As the new feathers grow in, the head can look scruffy for some time as it is difficult for the bird to self preen this area to remove the wax covers.  
Another possible cause is feather mites infestation, especially in young birds.  Look inside any bird nests and you will see they are literally crawling with mites & lice.  Once fledged, birds usually keep this in check through preening and good nutrition.

There are also records of individual birds in species such as Mynahs, Jays & Magpies that moult their head feathers all in one go, year after year, with no apparent reason why.   

Hope this info is of some help.

Raptor & Bird Rehabilitation Section Head