CanadaMark
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That looks exactly like heat distortion or atmospheric distortion to me, which can happen at any time of day, and even differ from shot-to-shot as the subject travels through the air. A slow moving bird at that distance is not a challenge for the AF system and DOF isn't a factor at F10. It can happen anytime. It doesn't even really look truly OOF to my eye, the edges look fairly well defined - it looks like the camera focused just fine, but through haze which destroys all the fine detail and makes it look 'mushy'.
At that distance I also doubt your shutter speed was an issue, it doesn't look like it and also you would typically see some parts of the image sharper than others if that was the case (i.e. sharp face but blurry wingtips). That image has the same level of 'sharpness' across the whole subject. Of course high ISO, low light, and simply not having many pixels on the subject are all working against you here but I don't think that is why the image looks the way it does.
Almost all my shooting these days is done at 800mm, so not much different than what you have there, and I've come home with sets of images that look exactly like that, and it was clearly heat haze. Morning, afternoon, evening - doesn't matter, you can still have atmospheric issues anytime there is a temperature differential. I probably notice it the most above water or around snow, this time of year anyway.
One way to quickly check for heat haze, especially if it's bad, is to defocus your lens and look through the EVF at the now blurry image preview - you can actually see the waves in the air that aren't as easily visible with the naked eye or when focused on a subject.
Hard to say 100% for sure just based on a couple photos, but again, that looks precisely like my images when they are affected by atmospheric issues.
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