Jay I don't really know but I think it depends on the type of flower. The flowers that stay on the stem for several days replenish nectar. Some flowers drop of each day.

Jay--- This is a rather complex subject which I have often wondered as Im sure many others have. What mimidi says is also true. Some plants produce much more than others concerning nectar or sugary substance , which after reading the article Im going to attach I find there is much more to nectar than just sugar. Also there is the "photosythesis" the process of turning light energy into chemical energy for the plants needs and the plant does this at night after the storing process. I also get from this article that some plants will produce nectar as needed. In other words the more a plant is used the more nectar it will produce. Im nothing even close to a botanist so read the article , it may help. I only know I see hummers using the same plants over and over. I also have read [dont know if this is true] that a hummer will first check the consistancy of a flowers nectar and if it is not yet up to proper sugar level that it will not take it , but will come back later and get it.

Here is a link that might be useful: about nectar

Steve thank you for you post and your site about nectar.

Thank you Jay for asking the question and thank you Steve for the info. I too have wondered the same particularly this week. I'm in zone 5, Milford, Mich. We have 1 female hummer left. She is ignoring my feeders but feeding constantly a the black and blues and lady in red salvias. I questioned whether she was getting enough nectar from them rather than hitting on the feeders. This week will dip and I anticipate a frost, I hope she can sense the weather coming and will leave in enough time, although I sadden when they're gone. Thank you again.

Im sure some of these cooler nights will push the hummers farther south. Last nights low here was 45 and after looking at the longrange not seeing any temps going below 40, but I live in a rural low lying area and anything could happen here. But as of today I have seen hummers on my feeder.