Cameras can easily get fooled in reading the light, especially if the subject is small and backdropped by light areas as birds high in trees or flying often are.
Setting a camera in manual and learning how to set exposure could be beneficial to many folks and would lead to a greater understanding of the automated systems.
Takes me a moment to take a look at the direction I'm shooting, point the camera to an average colored, large subject in the same direction with the same light falling on it (this is usually very easy, and pointing the camera at the back of a hand does it) and noting the exposure. I then just set that exposure manually. It takes seconds to do, much faster than writing it or reading it makes it seem. Then, the camera will not, cannot get fooled, and you can shoot away.
As for the "sweet spot of a lens", most likely the amount of background blur you want (usually the most possible for the least distraction when the viewer is looking at the shot) and image shake considerations will be more important than if you were at the sweetest aperture. To test this just put your camera on a tripod, focus on a certain item, and shoot at the various apertures. Look at the shots, now you'll know in the future if you wish to shoot that lens "wide open". Often too much ado is made regarding "this lens is sharpest at f8" type statements.
I'm not a bird photographer, so I'm not presenting myself as an authority on this (I took my first bird shot this week and it's only an "identification" shot, but the photographic advice is relevant). In this shot the displayed image is only about 1/5th of the frame, and even withing this cropped area the bird is only 1/4 of the frame, at most, so the camera's automated metering surely would have so grossly underexposed the bird that the detail in the slight iridescent coloring most likely would have been lost in noise. With a manual setting the bird did not need to be lightened.