I have been interested in recording flowers, along with other forms of wildlife, for many, many years but it was only comparatively recently I discovered that in addition to the common birdsfoot trefoil there is an equally common greater birdsfoot trefoil (Lotus pedunculatus). I find it hard to believe that for so many years I may have been making a fundamental error; there are far more complex identification issues that one get easily wrong but surely not between these two?
The flowers of both species are very similar although in greater birdsfoot trefoil they are, perhaps, a slightly dull yellow whereas in common birdsfoot trefoil they verge towards an orange-yellow. The main and obvious differences are in the rest of the plant. It is a much larger plant, hence 'greater', a bit straggly and has larger, darker leaves.
Although quite different when you take a look at the two there is a key difference I have not mentioned. Common birdsfoot trefoil thrives in short grass on dry soils whereas its greater cousin likes damp conditions; ditches, marshes, damp woodland rides and so on, All in all, no excuse for making a fundamental mistake here I think!