Here's a notice from Noreen Geyer Kordosky, who does wildlife rescue and rehab.
Wildlife Rescue Centers in the Tucson area receive many calls each year from people who have been watching “their” hummingbird nest, and suddenly think the adult female is no longer coming to feed the nestling. Also they might notice she isn’t sitting on the nest at night. Whether or not to cut down the nest and bring it to a rehabilitation volunteer is the question. Unless one can maintain a full hour of uninterrupted observation of the nest, one is likely to miss a 15-45 second visit from the adult. Only the female feeds the young. Before intervening, please take the full time to watch the nest site. Nests on branches that have been accidently cut down can be wired back onto the tree. Call if you have questions: 743-0677(h), 240-2686 ©