Finches

This shot of the goldfinches  competing for the food from the feeder is interesting from the perspective of the finches general health.  You will see that the feet of the finch in the centre of the image are badly affected by a scaly swelling. 

There are two main causes of these sorts of leg lesions in British finches.
Chaffinch papillomavirus is a virus that can cause skin disease known as papillomatosis.  Alternatively, mites can cause a skin disease known as cnemidocoptosis. 

The diseases are not necessarily fatal and sometimes, the birds can spontaneously recover, particularly where the cause of the swelling is papillomatosis.  Unfortunately, if it's the other disease,  then it's a case of "who knows" as the disease progression of cnemidocoptosis in wild birds is poorly understood at present.

Infected birds become less agile and are obviously more prone to predation and entanglement.

The main thing that we humans can do is to regularly disinfect the feeders and to air them after cleaning before we refill them.