Marsh and Willow Tit are mainly seperated by call. Regarding plumage differences and body differences (hard with an untrained eye). The Willow has a larger head with a bull like neck and generally looks a more robust bird than a Marsh Tit. Marsh Tit has a Square (or slightly forked tail whereas the Willow Tit has a slightly rounded tail.
The black cap on a Willow Tit is dull whereas the Marsh is glossy. The Willow has a pale wing bar whereas the Marsh hasn't. The black bib under the neck is larger on the Willow Tit.
Here are some examples.
One of my photos of a Great Tit.
One of my Blue Tit photos.
An excellent photo by glsammy of a Marsh Tit followed by another excellent shot from Wilf of another Marsh Tit.
Note on the first photo the lack of the light colour on the wing, whiteish on the Willow, as you can see on the Willow Tit photo. Also note on the 2nd Marsh Tit photo the glossy black cap. the Willow's will be dull.
Another excellent photo by glsammy of a Willow Tit.
Here is another Tit to consider. The Coal Tit. Following is a photo of mine of this species. Similar colours to Marsh and Willow but note the white crown stripe and the wing bars on this bird.
Hope that helps.
John
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