This squat little fellow is a Eurasian Bullfinch (Pyrrhula pyrrhula), a pudgy bird also known as the Common Bullfinch and found throughout Europe and throughout most of Asia. While the first thing that might strike someone upon seeing this bird is its plump build and stocky beak and neck, researchers have recently been interested in the shape of a different feature of this bird.
Most songbird sperm seems to stick to one particular shape: pointed at the top, with a helix shape for the tail. This bullfinch is very different though, possibly the most different of any songbird tested so far. Eurasian Bullfinch sperm is fairly short and rounded, rather like the bird itself. There has been some suggestion that this unusual shape came from a lack of directional selection on sperm shape because of low sperm competition in this species. Basically, some scientists think that because multiple males aren’t always mating with the same females in this species, there hasn’t been anything weeding out less efficient sperm shapes. Male Eurasian Bullfinches simply drifted to the shape they have now, with nothing pushing for a particularly fast shape, like the normal helix structure. More data is needed before anything conclusive is decided though, so there’s an interesting question to be answered!