People who feed wild birds and those who keep birds are warned in this article to ensure:
1. The feeders are kept clean by regular removal of old food, washing and thorough drying of the feeder
2. Food is kept dry
These two steps will prevent food becoming mouldy and poisoning the birds - damp foods will rapidly grow fungi and some of those can easily produce mycotoxins which can be lethal. We have an extensive article on avian (i.e. bird) aspergillosis here.
The areas under the feeders should also be kept very clean and clear of fallen seed which can quickly go mouldy and cause respiratory problems for the birds and the people involved. In severe cases (e.g. immunocompromised people, fungal asthmatics, ABPA) this is especially important to such an extent that they should avoid going near the bird feeders altogether.
Good attention to these details will minimise the possibilities of making the birds or their human friends ill.