When feeding birds, ensure that food is not stale or rotten, as this can be far more harmful to the birds than overeating. According to Colorado State University, the digestive enzymes produced by a bird's pancreas may not react well to rotted food.
It is generally difficult to gauge how fat a wild bird is by its appearance. Most wild birds are so heavily covered in feathers that it is impossible to determine the body weight with a visual inspection.
The National Bird-Feeding Society recommends providing habitat, food, water and nest boxes to make a yard bird-friendly. It notes that birds more frequently visit feeders placed near larger trees and shrubs.