What species is this bird (found in Cousset, Switzerland)?
This is really hard to tell given the quality of the picture. What size was it?

Could be Emberiza citrinella, the so-called yellowhammer - a year-long resident in Switzerland. It is about 16 cm in size and its breeding season starts beginning to mid-April and males are advertising for females in a golden plumage.



Again, this is only a best guess and without a better quality image it can never be more. RHA pointed out in the comments that the head of the bird in your image appears to be darker than one would expect in E. citrinella, suggesting it could also be E. melanocephala - the black-headed bunting, Spinus spinus - the Eurasian siskin, or Serinus serinus - the European serin. But consider this:


The bird on your picture appears to have a somewhat dark head but it does not seem to be black (but with the resolution it is hard to tell). E. citrinella males have brown stripes on their head that can be more or less pronounced (see examples in Google search for yellowhammer) - blurred, this fits the brown head in your picture. 
In your comment, you give the bird's size as the size of a house sparrow which exactly fits E. citrinella.  
E. melanocephala males have a pitch black head and the range it can be found does not fit your location. There are reports of vagrants but in combination with the brownish head in your picture this seems somewhat unlikely to me.
Sp. spinus and Se. serinus are smaller (max. of 12 cm in length) than what you describe.
Se. serinus is a partial migrant in Switzerland which decreases the probability of finding it in April, compared to a year-long resident. Additionally, it also only has brown stripes on its head.


In summary, I think the the most parsimonious call is that the bird on your picture is E. citrinella. However, keep in mind that it could also be one of the other species mentioned above, with E. melanocephala - given your size description and the dark head - perhaps being the second most likely identification.