I believe you shot the pics of Grebes with a "long focal" lens...400 mm Right?
So, the Canon 1100D has an APS-C sensor, and this means the equivalent focal lens would be something like 540mm. Therefore, the slowest shutter speed you should use to avoid blur from camera shake should be "1/540" sec.
You had the 1100D on a heavy tripod, so you would expect to avoid camera shake - and normally this works fine. But with such a long lens, the effect of the vibrations due to the mirrors flipping up is drastically magnified and it is certainly possible that it introduces enough vibration to cause too much blur.
In your case, I would try to shoot at a faster shutter speed. Increase ISO (3200?) and try 1/1000 if the aperture allows it.
This is the first thing to try in order to eliminate blur from camera shake.
But your problem could also have come from the AF accuracy. On a bright sunny day you don't have any problem with AF accuracy. Good news! So the AF works fine. But in hazy/foggy conditions, the AF doesn't work as well. It hunts and misses focus - not surprising because the AF system is generally based on detection of contrast.
Of course I don't exactly know how the AF system of the 1100D operates but I imagine it is probably about the same used by a lot of modern cameras...
When the contrast has a sufficient level, the AF gives a good "sharp setting"...and therefore when the contrast's level is a bit to low, it can give a "wrong" setting.
In your case, when the light is a little bit dim and when the air is "hazy" (= low contrast) you should perhaps try to focus manually.
And the last tip I could suggest: change the setting of contrast. Your camera offers certainly a menu dedicated to "custom image settings" (or something like this). In this menu, I will find an option named "contrast". Go there and increase the contrast, then shoot the same scene in the same light conditions. Perhaps this will help the "AF" to find the accurate "sharp point"? If it works, the result in the pic' will PROBABLY not be what You expect! The white parts of the image will look like burned and the shadows will look too dark...but the aim of this new contrast's setting is to check how to get sharp focus.
If this works...then You know how to get a right focus.
- Do it.
- Now You have a sharp focus. Fine.
- Disable the "AF" (= set "manual") and don't touch anymore the focus ring!
- Then carefully set the contrast function back to the previous contrast setting
- Don't forget to crank up the ISO and the shutter speed (minimum = 1/(focal length * 1.6) or 1/1000 if there is still enough light)
Now the 1100D is set with accurate focus and contrast for "hazy" ambiance.
Release the shutter via the self timer.