Animals have an inherently higher worth in terms of energy, niche, process, consciousness.

Bacteria left unchecked, could wipe out a species due to it's crazy exponential growth factors. For example over running the immune system.

Lately we have had scientific observations of some or of illness killing many birds across many species.

In this case you clean the "watering hole" if you are going manage that artificial shared resource to prevent being part of the problem.

I don't know for sure, but off the top of my head I can come up with two reasons:

1. Bacteria can be dangerous (for the animals, for us)

2. Birds are beautiful and squirrels are adorable (esp. with their their fuzzy tails). A pool of sludge is repulsive. So, uhm, I like animal growth better for personal, subjective reasons :)

One isn’t inherently better than the other, but if you spend time making a platform for birds, you probably want to enjoy looking at them rather than let them die.