I'm wondering if anyone else with a sciencey mind has noticed this preference, or has hypotheses about it.
Here are my observations:
I have 2 identical feeders of the same age side by side on my balcony. I make my nectar from sucrose 1:4 with water in summer, and depending on how bad the weather gets in winter, I mix it 1:3 or rarely even 1:2. Most of my hummers appear to be the offspring of a lone male who practically lived at my feeder about 7 years ago when no one else had feeders in the area. Now there are many, but there are also other feeders.
I have noticed that when I wash and fill one feeder, the birds use the old one instead of the fresh. They sometimes fly up to the new one, approach a port, but apparently without tasting it, they leave and go to the other. They prefer the older feeder even if it only has a few teaspoon of nectar left in the tray.
I never leave food out long enough to become visibly spoiled, but in winter, a couple of weeks isn't unusual, sometimes longer. I imagine there is a small amount of alcohol. It doesn't seem evolutionary sound for hummers to be drawn to alcohol because that sensation of warmth (if there's enough to do that) is false. It would actually cause them to lose body heat.
I've experiemnted, and it seems they prefer old 1:4 nectar over fresh 1:3 nectar. If both feeders are changed at the same time, they don't seem to have a preference. If one feeder has fresh 1:3 nectar and the other gets fresh 1:4 nectar, they prefer the 1:3. If I switch the feeders around so the old nectar is where the new nectar was, they still find the old nectar. They will lick that feeder dry before starting on the newer feeder.
The only time I see them at the newer nectar when there is also older nectar is about 15 minutes before sunset when traffic is high, aggression is low, and the older feeder has several birds at it so the late comers have to use the fresher feeder.
The odor of spoilage attracts them
They are put off by the scent (or taste) of doshwashing liquid (although I rinse VERY well.
They form such strong fidelity to specific productive flowers that they won't abandon them until they stop producing (i.e., the old nectar is dependable, so they have no interest in a new source). But this doesn't seem evolutionary sound either because I would think it best to have many sources.
So, anyone else observed this, or have some ideas? It's fascinating to me.