Did we first have swimming birds or flying birds? Looking at the swimming birds building nests just across my garden, I suddenly wondered how evolution came to swimming birds and whether flying birds started swimming or whether swimming bird like animals started flying. What came first and when and why?

Flying came first, as far as we know. The earliest known bird (currently), _Archaeopteryx lithographica_, already had aerodynamic feathers (Feduccia and Tordoff, 1979). The Solnhofen Limestone, where it was discovered, is ~145 million years old, so we can place the "when" flight evolved to greater than or equal to that time.

_Gansus yumenensis_ is regarded as the earliest aquatic bird and is dated from ~120 million years ago. The Hesperornithiformes are the sister taxon of _Gansus_ (less derived) and also aquatic. However, they date from ~85 million years ago. It is not clear whether the ancestors of Hesperornithiformes were also aquatic. Because they branched off the main line of birds before _Gansus_ , these ancestors would likely predate _Gansus_ , but those relatives have not yet been discovered.