Many people will repeatedly refill feeders as it is used up, often before the feeder is empty, leaving a portion of the food at the bottom of the feeder that is never eaten.
Many feeders are difficult for birds to completely empty and these tend to hold uneaten food for some time. Put that together with dampness caused by the ingress of rain (few feeders are waterproof) and you have the ideal conditions for the growth of fungi. Once fungi such as Aspergillus grow several things can happen
- they can start to produce highly toxic mycotoxins
- they can produce spores that can kill an unhealthy bird
- 1. can increase the chances of 2. happening!