Not to be semantically confused with : Bird's-Foot (Ornithopus perpusillus),
Orange Bird's-foot (Ornithopus pinnatus) (now extinct in the UK?), Bird's-Foot Clover (aka
Fenugreek) (Trifolium ornithopodioides), Hare's-foot Clover (aka
Hare's-foot Trefoil)(Trifolium arvense) nor with the true Trefoils (Trifoilium such as Hop Trefoil (Trifolium campestre), Lesser Trefoil (Trifolium dubium) or Slender Trefoil (Trifolium micranthum) - [all plants with similar names and belonging to the same Pea Family (Fabaceae) but in differing genera]
Easily mistaken for : Greater Bird's-foot-Trefoil (aka Large Bird's-foot-Trefoil) (Lotus pedunculatus) but that is twice as tall up to 1m, more robust, and has between 5 and 12 flowers atop a flowering stem, so if you find specimens with only two flowers, then it cannot be Greater Bird's-foot-Trefoil. It could also be mistaken for
Hairy Bird's-foot-Trefoil (Lotus suaveolens), but that is hairier with longer hairs.
Very similar to : Horseshoe Vetch (Hippocrepis comosa) which also has yellow flowers, but they are a deeper shade of yellow, and although often streaked with red, they are never suffused with red. It is shorter than Bird's-foot Trefoil and has multi-pinnate leaves.
There are other Bird's-foot Trefoils such as
Narrow-leaved Bird's-foot Trefoil (Lotus tenuis) but that has much narrower leaves or
Slender Bird's-foot Trefoil (Lotus angustissimus) which is both hairy, has between 1-3 flowers on a stalk only but is quite localised growing near the sea only in the south of England.
There is a variety :
Lotus corniculatus var. sativus which, un-like Common Bird's-foot-Trefoil, has hollow stems (as does Greater Bird's-foot-Trefoil) and is larger and more robust and which is often planted on roadsides and motorway verges and is probably introduced into the UK.
Some similarities to both : Dragon's-Teeth (Tetragonolobus maritimus) but that has solitary and pale-yellow flowers. And to Horseshoe Vetch (Hippocrepis comosa).
No relation to :
Bird's-foot Sedge (Carex ornithopoda) [a sedge with similar name].
Distinguishing Feature of Bird's-foot Trefoils in general : A large banner and a smaller pod-shaped cup of petals that look like just one petal unless prised apart to reveal a further hidden three, making the normal five altogether of a Pea-type flower.
Common Bird's-foot Trefoil is a perennial plant that grows in dry grassy places. It usually has bright yellow flowers (sometimes orange to red) in small bunches of between 2 and 7 (favouring 2-5) atop a wiry stalk. The flowers have the normal Pea-type structure with 5 petals, but because of the closed structure, it looks like there are but two petals, albeit one that is pod-shaped. Because of this, one of its common names is 'Eggs and Bacon' (although your Author has never seen bright yellow or orange bacon).
Being a member of the Fabaceae family, it is expected that Bird's-foot-Trefoils would harbour toxic Cyanogenic Glycosides, and indeed some specimens do, but others do not. Contrary to expectation and the commonly accepted role of Cyanogenic Glycosides in plants (to curtail the eating habits of insects upon the plant tissue) it was found by researchers that the larvae of the Common Blue butterfly (Polyommatus icarus) fared better feeding on the leaves and tissues of Bird's-foot-Trefoils that possessed the toxins than those that didn't! They grew fatter and larger! But the adult butterflies had no preference apart from the females which preferred
amino-acid rich nectar (whilst the adult males showed indifference to amino-acid content).