Sandra

This looks like it could be a species in the Funariid family, maybe a spinetail. It could be one of several species, and not knowing the location makes it much more difficult. If it is a spinetail, and if it was seen in northern South America, then my guess would be either Azara's Spinetail or Rusty-backed Spinetail. Neither of these is a perfect fit though, so those are more guesses than pronouncements.

Most field guides will point out rump color if it's an important field mark. Generally, two groups that are important to pay attention to rump color are warblers and finches. The rump plus back and wing pattern of white on shorebirds is often really helpful too, especially in flight. You can find a detailed explanation of appearance on the Birds of the World site. There are lots of images that help because they show individuals in different positions and plumage. Rump color and eye-rings are just two of the many features that we use to distinguish species. One of the Cornell courses gives a list of the main features to look for, and it's the combination that makes it easy to narrow down the list of possibles.

Azure-winged Magpies do not have barred wings. It's from a Eurasian Jay. Here's one in flight: https://birdsoftheworld.org/bow/species/eurjay1/cur/introduction

Check out American Goldfinches in non-breeding plumage. They don't really have a white eye-ring, but they do have a pale area around the face. They love feeders, and the wingbars and black wings making a V on their back sounds goldfinchy. They don't really have gray backs, but since you thought it might be a Least Flycatcher (unlikely since these guys are insectivores), I'm assuming there is some yellow and green tones. Goldfinches are common visitors to feeders.