A few years ago I was lucky to find some Pygmy Nuthatches feeding a pair of Mountain Bluebird chicks — an example of apparently rare behavior referred to as interspecific feeding. I encountered similar behavior (thanks to tips from Mark Rauzon and Frank Tona) recently at Lassen Volcanic National Park: some Red-breasted Nuthatches were feeding a pair (male and female) of Williamson’s Sapsucker chicks.
In the video below, a nuthatch is feeding a young, but much larger, sapsucker chick.
This video shows both species bringing food to the nest:
There appeared to be several nuthatches feeding the sapsucker nestlings, bringing food to the nest quite frequently. The sapsucker mother was also feeding the chicks, but not nearly as often as the nuthatches did.
For details and photos of my earlier encounter with this behavior, have a look at the blog post here.